Hagia Sophia
The Hagia Sophia is once again a working Mosque after 85 years as a museum. Of course, it's only 200 or so yards from the equally large Blue Mosque. This makes the Calls to Prayer very interesting...almost like Islamic Dueling Banjos.
Doorway Mantel in Old Town (Istanbul)
If only I could remember exactly where this was. Somewhere around the Hagia Sophia and Topkapi Palace.
Basilica Cistern (Istanbul)
Hagia Sophia Door Panels (Istanbul)
One of the advantages of Islam not allowing the depiction of human forms. They do really intricate scrollwork and patterns as part of their art and decoration.