The science museum (Franklin Institute) is very good. If you haven't been to Philadelphia before, you should get a cheesesteak from Jim's on South Street (or Pat's or Geno's, but they're in a less-nice area). You can see plenty enough Liberty Bell for me through the windows of the Liberty Bell Experience, without actually going in. You should also run up the steps of the Museum of Art, even if you're not going to go in (but going in's pretty good too).

Peter