Blue Mosque & Aya Sophia.
Breath takingly beautiful. So much history, life, and excellent people watching.
Be sure to bring a head scarf to the mosque, though if you forget they have some for you to wear, though it isn’t the most attractive thing. Spend time there and don’t miss out of the Topika Palace, I hear great things about it and we have to come back to Turkey just to see that.
cheers & happy traveling.