Hagia Sophia, the Blue Mosque and the Basilica Cistern, then a lokanta lunch and Gülhane Park at dusk.
Day 2
Bazaars & the Golden Horn
Grand Bazaar in the morning, Süleymaniye at midday, the Spice Bazaar and a sunset walk across Galata Bridge.
Day 3
Palaces & Bosphorus
Topkapı or Dolmabahçe, then a public ferry up the Bosphorus and dinner in Ortaköy or Beşiktaş.
Day 4
Asian side
Ferry to Kadıköy for the market streets, Moda’s seafront and an Anatolian lunch; return at dusk.
Day 5+
Go deeper
Balat, Fener and Eyüp Sultan, Istanbul Modern and Arter, or a full day on the Princes’ Islands.
Honest thought: Istanbul is bigger and hillier than it looks. Two days cover the old city; five let you cross the water, slow down and add a ferry day out.