if you like history, then you're in for a treat. my suggestions as a lifelong resident:
all of old city, which includes:
- idependence hall (take the tour) and the immediate surrounding blocks
-liberty bell (great shot looking past it towards independence hall)
-Delancey Street
-the cemeteries on pine street between 3rd and 5th streets
city hall -AMAZING building - at least go up to the top of the tower
reading terminal market
rittenhouse square
south street
love park, looking back at city hall
logan circle
rodin museum
art museum (look back at the city once you get up those steps!)
boathouse row at night (its behind the art museum, but you'll have to be on the other side of the schuylkill river)
ben franklin bridge, both day and night (NOT the walt whitman bridge)
great night time shots from penn's landing
if you can, get across the delaware river on camden's waterfront to get some shots of philly as the sun goes down.
that's all i have off the top of my head right now, if i think of more, i'll post them.
oh! also, as far as the cheesesteaks go, don't bother with pat's or geno's (no matter what you here). go to jim's on 4th and south. 
