Stopped off at Philadelphia 10 years ago as part of our 'grand tour' (basically had our lives, a tent and 2 bikes in the back of a car). Think we stayed for just over a day, which gave us the opportunity to run up the 'Rocky steps' and see the Liberty Bell amongst other things. Seeing the Liberty Bell was not without problem as we had our bikes parked out the front, and an over-anxious security guard decided they might be packed with semtex and made us go and move them...It's a very pretty place, and the river is very beautiful and well worth having a stroll next to.
Enjoy.