On everyone's must do list for Philadelphia is a trip to Independence Hall.
They run 30 minute tours from 9 am every day. These tours are free, but you can book online for a $1.50 booking fee per person, which is a great option if your tight on time and travelling during a busy peak.
You can walk around the grounds without a ticket, but you will still need to clear customs.
The tour takes you into three rooms in the Independence Hall, with a bit of the history of the building and the events that occurred there (the Declaration of Interdependence being one). The commentary seemed far and balanced with a lot of talk about the founding fathers feeling that it was good but not perfect (we found a great quote in DC to match this).
If you can't get tickets the buildings are worth the trip through security.
The Liberty Bell is conveniently located next door and can be viewed inside (you don't need a ticket but there can be a bit of a line) or as we did through the glass.