Were stayed in a hostel just steps from the Hagia Sophia, Blue Mosque, Basilica Cistern and a few other sites. They Hagia Sophia/Aya Sofya was massive and extremely impressive. The architecture was incredible, especially when taking into consideration it's age. Here's the wikipedia page on the building: http://en.wikipedia.org/wiki/Hagia_Sophia
The Blue Mosque was beautiful from the outside, but I felt the interior paled in comparison to it's neighbor.
The Basilica Cistern really surprised me as we kind of just stumbled about it. It was one of the cisterns for the city and is held up by over 330 columns all arranged in a grid. The symmetry was beautiful, and I can only imagine how cool it would have been to have traversed the chamber in a boat the way it used to be done.
The weather, while cold, wasn't unbearable and it was a nice chance for me to feel like I actually had a vacation instead of just a visit home. Turkey is definitely a country I need to see more of.