Philly-delphia is also known for "Scrapple" (which I wouldn't touch w/a ten foot pole but people swear by it). Seriously, I had clients from GB and they had hoagies and cheesesteaks and nearly fell over! You haven't eaten until you consume a few of those!
Of course there's the Liberty Bell, Betsy Ross' house, Independence Hall, Valley Forge and lots of historical sights. I'd rather talk about food, though...