Obviously, if you're into history you'll need to visit the Liberty Bell and Independence Hall.
Those who slept through their history classes may not remember that Philly was the Capital before Washington DC.
Anyway, after you get done taking pics for me and Harriett...I can recommend having a drink at this place and walking in the footsteps (and the spittle) of our founding fathers.
http://www.citytavern.com/history.html
Or, if you have a travel partner, the Tavern has excellent food.
Oh...and if you really have a lot of time, scope out possible locations for visiting Bills fans to stay in case we play the Eagles in Philly in 07.