If you have two days in Istanbul
Two days cover the historic centre and the Bosphorus. They do not cover more — and trying is the classic mistake.
Day 1 — Sultanahmet on foot, starting early: Hagia Sophia, the Hippodrome, the Blue Mosque and the Basilica Cistern. Lunch in the neighbourhood, the afternoon at Topkapı. If there is anything left in you, sunset from the Galata Bridge.
Day 2 — The Spice Bazaar in the morning as it opens, then a Bosphorus ferry to the Asian side. Afternoon in Kadıköy, with the market and the tables in the street. Back at dusk, which is the best crossing of the day.
What is left out: Cappadocia, the Princes' Islands and Ephesus. All are full-day programmes with travel attached; inside two days they eat everything else.