I will back the Tony Luke's recommendation--only forget about the whole cheesesteak thing and get the Italian sandwich (roast pork, broccoli rabe, and provolone). It is vastly better than a cheesesteak and could feed a family. One note, watch out for long lines if you happen to be going there for lunch on a day when the Eagles are playing.

The architecture of many neighborhoods of Philadelphia is worth seeing. The Art Museum neighborhood (roughly around 20th-24th, north of Spring Garden) is very cool, as are parts of University City (my favorite spot is Osage Avenue around 47th, but anything between 40th-48th and Baltimore to Spruce is neat). Germantown is also interesting. You may also want to take a ride out to Manayunk--cool shops and restaurants, although parking can be a bit of a nightmare.

There are a bunch of off-the-beaten path museums and places to visit outside the typical US History tour, such as Eastern State Penitentiary and the Mütter Museum.

Philly has a very good art scene: check out InLiquid for listings, as well as the Philly.com website for general entertainment.