Favorite thing: City Hall is a beautiful, old classical revival-style building that is so enormous that it takes up an entire city block. It is good to to take note of it's location if you will be walking around the city because it can be seen from just about everywhere and serves as an excellent landmark if you aren't sure where you are going! When we visited they were doing structural repairs to the building. A statue of William Penn, the founder of Pennsylvania, sits on top of the Hall's tower.
Favorite thing: There is just so much to do in Philadelphia. You can easily spend 3 to 4 days here and do something different everyday.