One of the best guides I found for all my questions on travel in Istanbul happened to be written by a hotel in Istanbul (Witt Hotel). It was surprisingly detailed, and so good that I'm surprised and grateful they left it available to non-hotel guests.
As my trip neared to a close, and my feet became completely numb, I decided to look for any and all non-active activities...hence a Cruise! I mean look how blue the Bosphorus is? I chose the short Şehir Hatları cruise based on recommendations from the Witt Magazine, and the tour departed across the street from a lot of the large attractions (Spice Bazaar, Sultanahmet, etc.)
If you ever go to Istanbul, please take this cruise at the beginning of your trip. You get to see all these amazing neighborhoods from the water that you now want to visit. But if you do what I did and only have 24 hours left, you are pretty much left planning a return trip!