My familiarity with Istanbul is extremely limited, informed only by a single layover. With nine hours between arrival and departure of my scheduled flights, I had roughly 4 1/2 hours to see the city. That is enough to see a good number of key points in the European core of the city, but obviously not enough to get an in-depth appreciation of something as big and wide-ranging as the Turkish capital.
Blue Mosque (Sultan Ahmet Camii) ♥♥♥ and Hagia Sophia ♥♥♥ are both extraordinary pieces of architecture, gorgeous inside and out. They also get really crowded very quickly, so the best opportunity to see them without crowds is to get in at the opening times. Mosaics on the upper level of Hagia Sophia are very worth exploring. Sultan Ahmet garden square ♥♥♥ bookended by the two grand mosques is one of the most photographed areas in all of Istanbul; other buildings on its perimeter include a hammam, a madrassah, and a couple of lesser places of worship.
Basilica Cistern (Yerebatan Sarnıcı) ♥♥ is also nearby. Due to renovation works it had no water in its reservoirs which according to my guide makes the sight less ambient, but even so it was still a pretty impressive place to visit, offering several unique details besides the underground reservoirs, such as upside-down Medusa head column supports.
Suleymaniye Mosque ♥♥♥ holds the most commanding hilltop position of all, and is fully restored (unlike other great mosques that are in various stages of seemingly perpetual renovation). It is beautiful and serene.
Nuruosmaniye Mosque ♥ is among the lesser sights in town, but very much worth a stop – it is a rare mosque in the Baroque style, understated and elegant.
Grand Bazaar ♥ is really a whole city in itself where you can get lost without a map. There are over four thousand stalls selling various stuffs but the stock inventory tends to repeat itself every few doors down (sweets, spices, souvenirs, tableware, lamps, carpets). Not without appeal but quickly becomes trivial. Decorations of interior spaces are an additional highlight.
Egyptian Markets ♥ by the Galata Bridge are a smaller version of the Grand Bazaar – only a couple of “streets” – but with more pronounced and uniform decorations of the interior.
Galata Bridge ♥ is a sight in itself. Fishermen along its sides are one highlight, and the lower level lined with restaurants is another.
Topkapi Palace is one major sight that does not really fit into the timeframe of the layover, as it requires a couple of hours all on its own. It is definitely a major point of interest in town.
Throughout the central area of Istanbul, there are rooftop cafes ♥ offering sweeping panoramas of the city. Make use of them.
I have to admit that the central streets connecting the sights did not struck my fancy all that much, but that could be a function of the very short visit. I will eventually give Istanbul the time that I normally give to major European capitals and improve both my appreciation of the city and this travel guide.
Logistics
The new Istanbul airport is located a fair distance from the city center. It is also huge, requiring non-trivial walks to get between the gate areas and the public arrival/departure areas. If you are considering a layover for exploring Istanbul, you cannot expect to reach the city sooner than about an hour and a half after landing; and you need to plan to start the trip back to the airport about 2.5 hours before the departure of your outgoing flight.
When your time is short and you want to see a lot, it is highly recommended to arrange for a private guide. The key benefits of that are the transportation to and from the airport would be arranged for you so you don’t have to worry about it, and also getting into the paid attractions (Hagia Sophia, Basilica Cistern) bypassing the lines. Of course, those who always prefer to listen to a narration while touring will get wealth of information. The guide that my family used on two separate occasions in 2019, Sefa Ozdemir ♥♥♥, got the highest marks from us on all counts.