You’ve got a good start. I spent hours in the Blue Mosque just trying to come to terms with its beauty and hugeness. Leave yourself enough time to dawdle. I thought it was pretty amazing they let in non-Muslims, and wonder if that’s going to be the case ten years from now.
The Basilica Cistern is very close to the Aya Sofia and Blue Mosque. It’s an enormous underground hall made by the Romans from old temple columns and Medusa heads and the like. It can be taken in pretty quickly, and I found it really impressive. There’s not much Roman stuff around. The aqueduct is probably only worth the hike for a Rome buff.
Chora Church is far from the center of the old city, near the Roman walls, but there are tourist buses right to it. Well-preserved Byzantine frescos and mosaics. What the inside of Aya Sofia is supposed to look like.
Near the Spice Bazaar is an old mosque called (I think) Rustem Pasha that is smaller than the Blue Mosque but much more ornately tiled inside.
I would recommend taking an evening ferry in the Golden Horn, even if it’s to nowhere and back. The city looks beautiful at night.
Enjoy spending millions!