With only one day to explore İstanbul you will only be able to focus on one or two major attractions.
The most obvious bet would be Topkapı Palace where you could easily spend most of the day, especially if you decide to have lunch there. If you have any energy left afterwards you could also pop into the Blue Mosque (Sultanahmet Cami) and take a turn around the historic Hippodrome or Gülhane Park, places without opening hours to worry about.
If you’ve already seen the palace and the weather is good you could instead spend a happy day on the long Bosphorus boat ride that will take you as far as Anadolu Kavağı whence you can climb the hill and gaze out over the Black Sea from beside Yoros Kalesi (castle).
Alternatively, you could pick one or two of the other historic sites of Sultanahmet – Hagia Sophia, the Yerebatan Cistern, the Archaeological Museum and the Great Palace Mosaic Museum (closed in 2024) – as they are all handily close together.
Shopping
If you want to devote your day to shopping there are several obvious options. It would be easy, if exhausting, to spend the whole day in the Kapalı Çarşı (Grand Bazaar) although that’s a rather touristy experience.
More manageable are the Mısr Çarşısı (Egyptian or Spice Market) at Eminönu or the Arasta Bazaar beside the Blue Mosque. If you visit either of these you will still have at least half a day left for other sightseeing.
Eating
If time is tight and you don’t know the city well you might want to eat in Sultanahmet rather than venturing further afield and having to worry about transport connections.
For something fairly authentic and not too expensive try Tarihi Sultanahmet Köftecisi Selim Usta on Divan Yolu which has been dishing up meatballs to enthusiastic locals since 1920.
For something more special try the rooftop Fine Dine Istanbul restaurant at the Hotel Arcadia Blue in Dr İmran Oktem Caddesi, Tel: 0212-516 9696 where the view is simply breath-taking.