You've gotta have at least one cheesesteak before you leave Philly. We were told by a local that Jim's was the place to go. When we got there at lunchtime, there was literally a line out the door. The line moved pretty fast, though, and we got our food about 1/2 hour after arriving. You can order your cheesesteak with American, Provolone, or Cheesewhiz... and then there are extras like mushrooms, onions, peppers, ...
I had mine with provolone, mushrooms, and onions. It was yummy.
If you're really, really hungry you can attempt to break the cheesesteak-eating record: 11 sandwiches in 1 1/2 hours!!! Good luck!