Table of Contents
- The Big Sights - What to See in Istanbul First
- Istanbul Activities for Adults Who Want to Go Further
- Unusual Things to Do in Istanbul
- Places in Istanbul: A Neighbourhood-by-Neighbourhood Guide
- What to Eat - Food is a Serious Istanbul Activity
- The History Underneath All of It
- The Bosphorus - Getting Between Continents and Sunset Cruises
- Art and Culture - What's Worth Seeing Beyond the Big Monuments
- Places to Go in Istanbul That Most Guides Skip
- Fun Things to Do With a Day or Two Left
- A Few Practical Notes for Visiting Istanbul
- Frequently Asked Questions
Istanbul's got a way of making every other city feel slightly boring by comparison. It's split across two continents - Europe on one side, Asia on the other, with the Bosphorus running right through the middle. There's Ottoman architecture sitting next to Roman ruins, ferry boats weaving past tankers, and a food culture that'll keep you genuinely busy for days. If you're trying to figure out what to do in Istanbul, this guide covers the full picture: main historical sites, local favourites and a good number of things most tourists miss entirely.
One thing that surprises people visiting Istanbul for the first time is just how many layers there are to it - Byzantine, Ottoman and modern Turkish history all sitting on top of each other, sometimes in the same building. It's an amazing city in the most literal sense: there's simply a lot here, and it rewards time spent wandering just as much as hitting the official list.
The Big Sights - What to See in Istanbul First
These are the places in Istanbul you actually should visit, especially on a first trip. They get busy, but there's usually a good reason for that.
Hagia Sophia
Hagia Sophia is the place that stops most people in their tracks. Built in 537 CE as a Christian church, it held the title of the largest building in the world for roughly a thousand years - which tells you something about the ambition behind it. The scale of the interior is pretty hard to prepare for: the central dome seems to float above you, and the light coming through the windows around its base is genuinely extraordinary. Hagia Sophia served as the cathedral of Constantinople for nearly a thousand years, then became a mosque after the Ottoman conquest in 1453, then a museum in the 20th century, and it's a working mosque again since 2020.
Visiting Hagia Sophia requires modest dress - covered shoulders and heads for women - and it's free to enter outside prayer times. Get there early if you want any chance of seeing it without crowds. As an architectural masterpiece that's bridged two major world religions and multiple empires, Hagia Sophia sits in a category of its own.
The Blue Mosque
Hagia Sophia and the Blue Mosque face each other across Sultanahmet Square, which makes it easy to visit both in the same morning. The Blue Mosque - officially the Sultan Ahmed Mosque - was built in the 1610s and is the only mosque in Istanbul with six minarets. Inside, the walls and ceiling are covered with over 20,000 hand-painted blue Iznik tiles, which is where the name comes from.
The Blue Mosque is still an active place of worship, so it closes to visitors during prayer times (five times a day). Cover your shoulders and head before you go in and expect a queue if you're there between 10am and 3pm. Both the Blue Mosque and Hagia Sophia are right in the city center, so they're easy to combine with the Basilica Cistern in the same few hours.
Topkapi Palace
This is the one most first-timers underestimate. Topkapi Palace is enormous - basically a small city on a hill above the Bosphorus - and it served as the administrative center of the Ottoman Empire and the main residence of the Ottoman sultans from the 1460s right through to the 19th century. There's loads of genuinely remarkable stuff inside: the imperial treasury, tiled courtyards, the harem quarters and views over the water that explain immediately why the sultans chose this spot. Budget at least three hours. The Istanbul Museum Pass covers entry, which makes it much better value than paying separately for each site.
Grand Bazaar
The Grand Bazaar is one of the oldest and largest covered markets in the world. It covers 61 streets and contains over 4,000 shops - everything from carpets and ceramics to gold jewellery and leather goods - and on a busy day it pulls in somewhere between 250,000 and 400,000 visitors. That's a lot of people in a relatively small space, so go early or on a weekday morning if you want to move freely. The Grand Bazaar is genuinely worth wandering even if you're not planning to buy anything - the architecture alone, with its painted vaulted ceilings and centuries-old arches, is reason enough. It's also where you'll find some of the best Turkish delight in the city, sold loose by weight.
The Spice Market (Egyptian Bazaar)
A short walk from the Grand Bazaar, near Eminönü, is the Spice Market - also known as the Egyptian Bazaar or Mısır Çarşısı. It's smaller and more focused, with stalls selling spices, dried fruits, Turkish delight, teas, nuts, soaps and Turkish towels. The spice market is best visited early in the morning before the tour groups arrive - the light inside and the smells from the stalls are actually something, especially around 9am. It's closed on Sundays and certain public holidays.
The Iconic Galata Tower and Galata Bridge
The iconic Galata Tower has been a fixture of Istanbul's skyline since it was built in 1348. From the observation deck at the top, you get a panoramic view over the old city, the Golden Horn and the Bosphorus - it's one of the better viewpoints in the city and pretty accessible from Karaköy below. Galata Bridge connects Eminönü to Karaköy, and it's always lined with locals fishing off the sides - which is honestly one of the more memorable things about the whole area.
Basilica Cistern
The Basilica Cistern is one of the best places to visit in Istanbul on a hot afternoon - an underground Byzantine reservoir built in the 6th century using 7,000 slaves and featuring 336 columns rising out of shallow water. It's properly atmospheric, and a little eerie in the best way. The Basilica Cistern served as a main water source for the great palace of Constantinople and is just one of several hundred ancient cisterns that run under Istanbul's streets - though it's the most visited.
If you want something quieter, the Binbirdirek Cistern (also known as the Cistern of Philoxenosis) nearby gets far fewer visitors and is just as old. Both are worth seeing - the Basilica Cistern for the atmosphere and historical significance, Binbirdirek for the peace and quiet.
Süleymaniye Mosque
A bit uphill from the Grand Bazaar, Süleymaniye Mosque was built in the 1550s and is one of the largest mosques in Istanbul. It's in remarkably good condition for a 500-year-old building - the architect Sinan designed it to last, and it shows. The courtyard and gardens have pretty good views over the Golden Horn and the European side of the city. Less crowded than the Blue Mosque and Hagia Sophia, and well worth including on any visit to the city center area.
Dolmabahçe Palace
Where Topkapi is sprawling and ancient, Dolmabahçe is 19th-century Ottoman baroque - ballrooms, chandeliers, a 36-metre entrance hall. Completed in 1856, it replaced Topkapi as the main administrative center of the later Ottoman Empire and sits on the Bosphorus waterfront between Karaköy and Beşiktaş. Tours are guided only, so arrive early. The gardens and Bosphorus-facing facade are worth seeing even if you don't go inside.
Istanbul Activities for Adults Who Want to Go Further
Get on a Ferry
This might be the single best thing you can do in Istanbul, and it costs almost nothing. There are 22 public ferry lines running along the Bosphorus, with fares based on distance. The main routes connect Karaköy, Kabatas, Beşiktaş and Eminönü on the European side with Uskudar and Kadikoy on the Asian side. Ferries run until 11pm, so sunset on the water is genuinely an option - and a much cheaper one than a private sunset cruise. The views of Istanbul's skyline from the water are hard to replicate any other way.
Spend a Morning in Kadikoy
Kadikoy is on the Asian side, and it's where a lot of Istanbul's younger, local crowd actually lives and eats. The market area around Kadikoy Market is good for produce, cheese, olives and general wandering. The neighbourhood's got a relaxed, slightly scruffy energy that the European side's tourist areas don't really have. Get the ferry from Eminönü or Karaköy - it's about 20 minutes.
Fener and Balat
These two old neighbourhoods in the western part of the European side get mentioned a lot but rarely covered properly. Fener's the historic Greek quarter, Balat was Istanbul's Jewish neighbourhood for centuries. What you actually get here is narrow streets, wooden houses in various states of repair, small lokanta-style restaurants and a genuinely unhurried atmosphere. Balat in particular has loads of charming cafes tucked into old buildings, and the colourful painted houses along some streets are pretty distinctive. It's a good few hours, and very different from the Sultanahmet tourist zone.
A Traditional Turkish Bath
A traditional Turkish bath - a hamam - is one of those things that feels like a cliché until you actually do it, and then you understand immediately why people have been coming to them for centuries. The Hürrem Sultan Hamami (also known as the Hagia Sophia Hurrem Sultan Bathhouse) is right between Hagia Sophia and the Blue Mosque - built by Sinan in the 1550s and one of Istanbul's best preserved. The Cağaloğlu Hamami nearby is one of the oldest still operating in the city. Both offer the full traditional Turkish bath experience: a scrub, a foam massage and time on the heated marble platform. It's an ancient tradition and honestly a pretty good way to spend a cold afternoon.
Ortakoy
Ortakoy sits right at the foot of the Bosphorus Bridge between Beşiktaş and the city's wealthier northern neighbourhoods. The Ortakoy Mosque is photogenic - it's perched right on the waterfront with the bridge behind it, and it's open to non-Muslims. The area's known for kumpir (stuffed baked potatoes with a lot of possible toppings) sold from street stalls, and on weekends there's usually a market with boutique shops and street performers along the waterfront.
Çukurcuma
This is the antiques neighbourhood, roughly between Taksim Square and Cihangir. Small shops packed with old furniture, vintage clothing, retro objects, maps, tiles and all sorts of things. Serdar-i Ekrem Street nearby has a mix of boutiques and independent shops. If you're after vintage shopping or antique furniture hunting, this is where to come - and there are good quaint cafes nearby to stop into between shops.
Taksim Square and Istiklal Avenue
Taksim Square is the main public square of modern Istanbul - the Republic Monument at its centre marks the founding of the Turkish Republic. Istiklal Avenue (also known as Istiklal Street) runs from Taksim Square down towards Karaköy through a kilometre-plus of shops, art galleries, cafes and restaurants. There's live music coming out of various venues on most evenings and street performances most weekends. The Tünel funicular at the bottom - the world's second-oldest underground railway - connects Istiklal Street down to Karaköy. Cicek Pasaji, a 19th-century arcade off Istiklal Avenue, is slightly faded but still worth a look. And there's an old tram that runs the full length of the avenue - slow and packed, but atmospheric.
Unusual Things to Do in Istanbul
Some of the best things to do in Istanbul aren't on any official list.
Sand Coffee and Coffee Grounds Reading
Turkish coffee is well known. Sand coffee - where the cezve sits in heated sand and brews slowly - is a bit harder to find but worth seeking out. Some cafes around Sultanahmet and the bazaar area still do this properly. Afterwards, you can get a coffee grounds reading (tasseography) where someone reads the grounds left at the bottom of your cup - it's called falıcılık and it's a genuine cultural practice, not a tourist gimmick.
Dawn at the Spice Market Area
Eminönü at dawn, before the tour groups arrive, is a completely different place. The grilled fish sandwich boats (Balik Durum) are already going, the ferries are loading, there are vegetable sellers along the waterfront. It's the Istanbul that most visitors miss.
Rooftop Cafes With Views
A good number of cafes and restaurants in Sultanahmet, Karaköy and Galata have rooftop terraces that aren't obvious from street level. Some have direct views of Hagia Sophia and Istanbul's skyline that you really can't get at street level. Worth asking about, or just exploring upstairs when you see a staircase.
Bike the Theodosian Walls
The Walls of Constantinople - built in the 5th century and still largely standing on the western edge of the city - stretch for several kilometres. You can hire an e-bike and follow them, which is a pretty dramatic way to appreciate just how serious Byzantine city defences were.
Whirling Dervishes Ceremony
A Whirling Dervishes ceremony is one of Istanbul's more unexpectedly moving things to see. The dervishes whirl in long white robes to live music and chanting - it's a Sufi spiritual practice, not a performance as such, though it's held at various venues around the city including cultural centres and mosques. Checking what's scheduled during your visit is worth the five minutes it takes.
Prince's Islands Escape
A short ferry ride from Kabatas or Eminönü gets you out to the Prince's Islands in the Marmara Sea - a genuinely good escape from the city. Büyükada is the largest and most visited: no cars allowed, so it's horse-drawn carriages and bicycles through lush pine forests with beaches below. Heybeliada, Burgazada and Kınalıada are quieter if you want fewer people and more fresh air. The Prince's Islands are particularly good in spring and early autumn - summer weekends get pretty packed with locals from Istanbul.
Pierre Loti Hill and the Golden Horn
Take the cable car up to Pierre Loti Hill above the Eyüp neighbourhood for views over the Golden Horn. It's a tea garden at the top, essentially, but the view down over the mosques and waterway is unlike anything else in the city.
The Hippie Trail Connection
Istanbul was the starting point of the old Hippie Trail east to Asia, and the Lale Restaurant - also known as the Pudding Shop - on Divan Yolu in Sultanahmet was where travellers used to pin notes and messages for each other. It's still there and still a perfectly decent place to eat, though the bulletin boards are long gone.
Places in Istanbul: A Neighbourhood-by-Neighbourhood Guide
| Neighbourhood | Where | Best for |
|---|---|---|
| Sultanahmet | European side, city center | Hagia Sophia, Blue Mosque, Grand Bazaar, Basilica Cistern, Topkapi Palace |
| Karaköy | European side, waterfront | Ferries, coffee, iconic Galata Tower, Istanbul Modern |
| Galata / Cihangir | European side, hilly | Çukurcuma antiques, boutique shops, quaint cafes |
| Taksim / Beyoğlu | European side, central | Taksim Square, Istiklal Avenue, art galleries, live music, Pera Museum |
| Beşiktaş | European side, waterfront | Dolmabahçe Palace, Yıldız Park, local food |
| Ortakoy | European side, Bosphorus | Ortakoy Mosque, Bosphorus views, street food, street performers |
| Eminönü | European side, waterfront | Spice Market (Egyptian Bazaar), ferry terminals, grilled fish sandwiches |
| Kadikoy | Asian side | Local cafes, market, food scene, ferry views |
| Uskudar | Asian side | Maiden's Tower views, quieter mosques, local life |
| Fener / Balat | European side, western | Historic streets, wooden houses, charming cafes |
What to Eat - Food is a Serious Istanbul Activity
Istanbul's food scene is pretty deep if you actually look for it rather than eating wherever's nearest to a landmark. A food tour is a solid option if you're only in the city for a couple of days - they typically cover four or more stops across a neighbourhood and give you a decent introduction to what's actually worth eating. But it's also fine to just walk and stop at whatever looks good.
Turkish Breakfast
If you only do one traditional Turkish meal, make it breakfast - it's its own thing here, not just toast and coffee. A full spread includes local cheeses, olives, honey, fresh bread, tomatoes, cucumber, eggs and small dishes of various spreads. Plenty of restaurants around Karaköy and Kadikoy serve it properly.
Lokanta Restaurants
These are the Turkish equivalent of a workers' canteen - proper home-cooked food served from trays at very low prices. Hayata Saril Lokantasi, Balkan Lokantasi and similar places in the Eminönü and Karaköy areas are worth finding. You point at what you want, pay about the cost of a coffee in most European cities, and the food is genuinely good.
Cag Kebab
This is a vertically-spit-roasted lamb kebab from eastern Turkey. Şehzade Cağ Kebap in Sultanahmet is one of the better-known spots for it in Istanbul - different texture and flavour to the usual döner.
Grilled Fish Sandwich
The grilled fish sandwich - Balik Ekmek in Turkish - is one of Istanbul's most iconic pieces of street food. Sold from boats and stalls around Galata Bridge and Eminönü, it's cheap, good and unmistakably Istanbul.
Meze
Small dishes - dips, salads, stuffed vegetables, cold seafood - served at the start of a meal in meyhanes (taverns). Particularly good around Karaköy and Kadikoy.
Street Food Basics
Simit (sesame-crusted bread rings) are sold from carts everywhere and they're excellent. Kokoreç (grilled offal in bread) is a late-night street food that's not for everyone but very much part of Istanbul's food culture. Börek - filo pastry with cheese or meat - is good from almost any bakery. Kumpir from Ortakoy is worth a separate trip.
Baklava and Turkish Delight
Every neighbourhood's got pastry shops. Turkish delight - lokum - is sold in both the Grand Bazaar and the Spice Market, in a range of flavours that go well beyond what you'll know from elsewhere.
Turkish Coffee and Tea Culture
Tea (çay) comes in small tulip glasses everywhere and gets refilled constantly. Turkish coffee is thick, unfiltered and served with a glass of water. For something more unusual, look for sand coffee where the cezve brews in heated sand - a slightly slower, more theatrical take on the same drink. And if the cafe offers to read your coffee grounds afterwards, say yes.
The History Underneath All of It
Istanbul's been continuously occupied for around 3,000 years - as a Greek colony, as Constantinople (capital of the Eastern Roman and then Byzantine Empire) and as Istanbul, capital of the Ottoman Empire from 1453. The Republic Monument on Taksim Square marks the Turkish Republic period from 1923. For history buffs, honestly, you could spend a week here and still have more to see.
Hagia Sophia sits at the centre of all of it - originally built as a Christian church in 537 CE, it became the defining building of the Byzantine world and then a mosque under the Ottoman sultans. Its historical significance is hard to overstate. What this means practically is that there's Byzantine architecture sitting underneath Ottoman additions, and Roman structures underneath those. The Theodosian Walls are 5th century. The Hippodrome in front of the Blue Mosque was Constantinople's chariot racing track, and the Egyptian Obelisk and the Serpent Column still stand there.
The Archaeological Museums near Topkapi Palace cover this in decent depth - three separate buildings with collections ranging from the Neolithic to the Ottoman period. The Museum of Turkish and Islamic Art is also worth an afternoon if you've got any interest in carpets, calligraphy or decorative objects. These historic buildings collectively make Sultanahmet one of the most historically concentrated areas in any of the world's major cities.
The Bosphorus - Getting Between Continents and Sunset Cruises
The Bosphorus is the strait connecting the Black Sea to the Marmara Sea, and it's one of the busiest shipping channels in the world. It's what makes Istanbul the only city spanning two continents. It's maybe 30-35 kilometres long and narrow enough in places that you can see the Asian side clearly from the European side.
The public ferry system is genuinely one of the best ways to see the city - routes run between Eminönü, Karaköy, Kabatas and Beşiktaş on the European side and Uskudar, Kadikoy and various other Asian-side stops. Ferries run until 11pm, which means sunset cruises on the public system are a real option - not just private boats. A longer Bosphorus cruise runs from Eminönü up towards the Black Sea and back, stopping at a few villages along the way. The views of waterfront yalıs (Ottoman historic mansions) from the water are pretty good.
For getting around: the Istanbulkart is a rechargeable card that covers all public transportation - metro lines, trams, buses, dolmus (shared minibuses) and ferries. It's much cheaper than paying cash for each journey. The Tünel funicular between Karaköy and Istiklal Street is included. Taxis are fine but make sure the meter's running.
Art and Culture - What's Worth Seeing Beyond the Big Monuments
Visiting Istanbul purely for its historic buildings is understandable, but the art and culture scene is actually pretty strong.
Istanbul Modern
The city's main contemporary art museum, recently moved into a new building near Karaköy. It's a serious art gallery with a strong permanent collection of Turkish modern and contemporary work, plus international temporary shows.
Pera Museum
Near Taksim Square, with a permanent collection of Ottoman-era paintings and Anatolian tiles and weights, plus rotating international exhibitions. Less crowded than the big historical sites and genuinely interesting.
Sakıp Sabancı Museum
On the Bosphorus in the Emirgan neighbourhood - calligraphy, Ottoman paintings, and usually a strong temporary exhibition programme.
Istanbul Archaeological Museums
Three museums in one complex near Topkapi - the main archaeological collection, the Museum of the Ancient Orient and the Tiled Kiosk. Underrated and genuinely impressive if you've got any interest in the ancient world.
Gülhane Park
Just below Topkapi Palace, Gülhane is one of Istanbul's older public parks and a decent peaceful retreat in the middle of the old city. Good for a walk between the Archaeological Museums and Hagia Sophia - it's right in the city center but feels noticeably quieter than the streets around it.
Yıldız Park
A large forested park above Beşiktaş with walking paths, old pavilions and views over the Bosphorus. Good for a morning walk if you're staying on the European side, particularly outside summer weekends. Nature lovers could easily spend a few hours here.
Places to Go in Istanbul That Most Guides Skip
Maiden's Tower (Kız Kulesi)
A small tower on an islet just off the Asian shore near Uskudar. You get to it by a short boat ride. It's been a lighthouse, a quarantine station and briefly a restaurant. The view back towards the European side - with Hagia Sophia and Topkapi visible on the skyline - is one of the better ones in the city.
Çamlıca Hill and the Çamlıca Mosque
Istanbul's highest point is on the Asian side. The Çamlıca Mosque was completed in 2019 and is the largest mosque in Turkey - worth seeing even if modern religious architecture isn't usually your thing. The views from the hilltop gardens over the whole city and the Bosphorus are genuinely clear on a good day.
Moda
A residential neighbourhood south of Kadikoy on the Asian side with a waterfront promenade, old architecture and a noticeably calmer atmosphere than the rest of the city. It's got good cafes and is particularly pleasant on weekend mornings.
Uskudar Waterfront
The Asian-side waterfront from Uskudar south has some of Istanbul's better Bosphorus views - looking back at Sultanahmet and Topkapi with the water in between. The Mihrimah Sultan Mosque here is worth the trip on its own.
Fun Things to Do With a Day or Two Left
By the time you've covered the main sights, here are a few things that are just enjoyable rather than historically significant:
- Vintage shopping through Çukurcuma and Cihangir - a few hours easily
- An evening at a meyhane in Karaköy or around Istiklal, with raki and meze
- Coffee grounds fortune telling session at one of the older cafes in Sultanahmet
- Walking the full length of the Theodosian Walls from the Golden Gate to the Golden Horn
- A Prince's Islands escape - afternoon ferry to Büyükada and back, swimming, cycling and lunch
- Watching street performances along Istiklal Avenue on a weekend evening
- Wandering the Bosphorus waterfront in Beşiktaş and Ortakoy on a weekend afternoon
A Few Practical Notes for Visiting Istanbul
Istanbul's pretty big and the traffic can be genuinely bad, so building your days around neighbourhoods rather than trying to criss-cross the whole city makes sense. The Istanbul Museum Pass covers most major state-run museums including Topkapi and the Archaeological Museums - worth getting if you're spending more than two or three days. The Istanbulkart covers all public transportation including ferries.
Currency's Turkish Lira. ATMs are everywhere, and Visa and Mastercard are accepted in most places but it's worth having some cash for smaller restaurants, markets and ferry tickets. A note on religious holidays: some sites and many smaller restaurants operate reduced hours during major Islamic holidays, so it's worth checking dates before you visit Turkey.
Istanbul's got two major airports - Istanbul Airport (IST) on the European side and Sabiha Gökçen (SAW) on the Asian side. IST is the larger one and handles most international traffic.
Tour options are easy to find for guided walks, cooking classes, ceramics workshops and Bosphorus sunset cruises if you want those pre-organised. But honestly, a lot of the best things to do in Istanbul are just things you find by wandering around - particularly in Balat, Fener and the streets around the Grand Bazaar, where the city still feels much older and more interesting than any guided route will show you.
Frequently Asked Questions About Things to Do in Istanbul
How many days do you need in Istanbul?
Three days covers the main historical sites in Sultanahmet reasonably well - Hagia Sophia, the Blue Mosque, Topkapi Palace, the Grand Bazaar, the Spice Market and the Basilica Cistern. Five days lets you add the Asian side (Kadikoy, Uskudar), the Bosphorus ferry properly, a Prince's Islands day trip and some of the quieter neighbourhoods like Balat and Fener. A week or more and you start to feel like you actually know the city a bit.
What's the best way to get between the European side and the Asian side?
The public ferries are the best option - they're cheap, frequent and the views from the water are genuinely one of the highlights of visiting Istanbul. Ferries run until 11pm. The main routes cross between Eminönü or Karaköy on the European side and Kadikoy or Uskudar on the Asian side. The crossing takes about 20 minutes.
Is Hagia Sophia free to visit?
Yes, entry to Hagia Sophia is free outside of prayer times. Since returning to mosque status in 2020, it operates as an active place of worship - which means it closes to visitors during the five daily prayer times. Modest dress is required: covered shoulders and heads for women. Get there early to avoid the main crowds, ideally before 9am.
What's the best neighbourhood to stay in for first-time visitors?
Sultanahmet puts you walking distance from the main historical sites - Hagia Sophia, the Blue Mosque, Topkapi Palace, the Grand Bazaar and the Basilica Cistern. It's the obvious choice for a short first trip. Karaköy and the Galata area are a bit more local-feeling, with better restaurant options and easy ferry access. Taksim Square area works well if you want to be closer to Istiklal Avenue and the newer parts of the city.
What should I know about visiting mosques in Istanbul?
Both the Blue Mosque and Hagia Sophia require modest dress - covered shoulders and heads for women, shoes removed at the entrance. They close during the five daily prayer times, which vary by season. The Süleymaniye Mosque and Ortakoy Mosque are generally less crowded and also worth visiting. Friday midday prayer sees heavier closures across all mosques. If you're planning your visit around religious holidays, check in advance as schedules can change.
Is the Istanbul Museum Pass worth buying?
Yes, if you're spending more than two days. The pass covers Topkapi Palace, the Archaeological Museums, the Museum of Turkish and Islamic Art, Galata Tower and several other sites. It also skips the ticket queues at most venues, which is genuinely useful at Topkapi and Hagia Sophia in peak season.