There are 3 million things to do in Istanbul. I’m going to start a running list of the things that sound interesting to me.  This way I can sift through them later and pick the ones that are MUST do and things that are MAYBE do.

• Topkapi Palace- location of the head of the Ottoman Empire for over 300 years.  Supposedly should take at least .5-1 day for this.
• Church of the St. Saviour in Chora (Chora Church)- a Byzantine Christian church with mosaics and frescoes that were amazingly preserved despite the Ottoman take-over
• Ferry across the Bosphorus – Don’t take a cruise, they are more expensive and not any better than the commuter ferries
• Chill out under the Galata Bridge – watch the boats, go to tea houses and restaurants
• Hagia Sophia
• Walk the Theodosius Walls 
• Dolmabahse Palace
• Hamam – find one the locals go to, they are cheaper and more authentic. Drink lots of water!
• See the Whirling Dervishes – This site looks interesting.  Do some more research though.
• Visit Princes Island
• Archaeology Museum
• The Blue Mosque
• The Grand Bazaar
• The Egyptian Spice Bazaar
• Mosque of Suleyman the Magnificent – 16th century
• Istiklal Street- pedestrian only street full of shopping and restaurants
• Underground Cistern
• Hagia Irene
• Galata Tower