Istanbul is the only city that sits on two continents — the Bosphorus splits Europe from Asia and ferries cross it every fifteen minutes, full of commuters reading newspapers and tourists trying to look casual about it.
Sultanahmet has the famous postcards (Hagia Sophia, Blue Mosque, Topkapı), but the city really opens up further out: tea gardens in Çukurcuma, Sunday brunch in Karaköy, the antique-spice rush of the Mısır Çarşısı. Walk a different neighbourhood each day and Istanbul keeps unfolding.
Two truths to hold together: it's enormous (16+ million people) and it's intimate (everyone has a tea-glass and an opinion). Plan less, wander more, and accept the offer when someone invites you to sit down.