I spent last Thursday in Philly (I had some Amtrak vouchers). I spent most of my time in the old city, which is great if you’re interested in the history of the Founding generation, which I am. The Liberty Bell is there, of course, and Independence Hall was very interesting (where they approved the Declaration of Independence and the Constitution). It’s flanked by the buildings where the Supreme Court and the U.S. Congress first met as well.
Within walking distance are lots of other interesting things such as the 2d Bank of the U.S. (which has a pretty cool portait gallery of the movers and shakers of that time period) and Franklin Court, where Ben Franklin had a house (now gone) and some townhouses he rented out. One of these houses exhibits of stuff excavated from the trash pits.
Ignore everyone who says you shouldn’t go to Pat’s and Geno’s because it’s for tourists. You are a tourist! It’s rather farther than you want to walk from the old city, but the #47 bus will take you there and back. (As much as I despise the Vento family’s recent political kerfuffle, I’ll be damned if I didn’t like Geno’s better.)
If you’re not going to have car access when you’re there, you can get a $5 all-day transit pass, which gives you unlimited rides on both the busses and the subway. But I found it very difficult to figure out what busses went where, so some pre-planning is in order. (Of course, with the day pass, if the bus you’re on turns in an unexpected direction, you can just get off and wait for the next one.)