I like Philly. Check out the Art Museum and boathouse row behind it is neat. The historic district is cool and they have an indoor market place (reading Terminal Market) that is freaking cool and has some pretty eclectic food...plus you can also get a cheese steak in one of the booths that is way better than either Pat's or Geno's. Ask a local for the best cheesesteak in Philly and none of them will tell you Pat's or Geno's anymore. Those are the tourist traps and they serve tourist trap quality food these days (I had a friend who was originally from just outside of Philly, lives in Boston now, who went back for a short visit and they went to Geno's and he posted a picture and was pretty sure they're serving up Steak-Umms now). Also, if you wallk up Arch St from the Liberty Bell/Independence Hall area, you'll pass Reading Terminal Mkt, and you'll come to the start of the museum mile starting with the Franklin Institute leading up to the Philadelphia Art Museum (the Rocky steps). Lots to see in there. Of course if you are a history buff, you could spend an entire weekend in Independence Sq. the Betsy Ross house is there, the Museum of the American Revolution, a cemetery with many notable Revolutionary era leaders resting places. Depending on how long you're there, you can venture up the Delaware River Valley and check out Valley Forge.
As Senior says, don't venture too far from the touristy things in Philly. Don't cross the river into Camden for sure, although Barry Goldberg is convinced the best cheesesteak around is over there
