Gary,
My then-girlfriend (now-wife) and I visited Philly for a few days several years ago. If you're at all a history buff, definitely get down to see not only the Liberty Bell, but have enough time to do the tours of all of the Independence Hall area. It's a very cool, almost eerie feeling to be in the room where they debated the Declaration of Independence and the Constitution, with the guide informing you where each Founding Father was seated at the time.
If you're looking for a more light-hearted, fun tour, check out the Ducks. They're as cheesy as it gets, but they're a lot of fun, and (from the perspective of this non-resident) provided a lot of great information about the city as well.
We had a lot of fun there--although Michelle was a bit disappointed going home because she thought I was planning to propose while we were at a Phillies game. (It happened instead about three weeks later while back at home.)