I have been to Istanbul, Antalya and Pamukkale. If you are into Islamic History then you will like a visit to Sultan-Ahmet Square. Blue Mosque was beautiful but under renovation when I visted. I liked Topkapi Palace and Hagia Sophia besides Basilia Cistern. These are all in the Square.

Couldn't Explore Antalya much. Only the Duden Waterfalls.

Pamukkale was my favorite among all. The Hike in the thermal pools was adventurous and at the top Hierapolis Museum was fascinating. I was so tired after the hike, couldn't visit all the places though. We did a day-trip from Antalya, a night stay would have been better.