Don't think I've seen it mentioned in the thread yet, so if your looking for something off beat and really cool for a young adult look into the Mutter Museum. We visited this museum last winter with 3 young adults and they still mention some of the things on display pretty frequently. Wana see the worlds largest colen? It's on display there.
This might not be fore everyone, but I know many people who will also recommend it. We visited ths museum and then cabed it to the reading terminal market for lunch. DeNicks pork is one of our favs. A way to beat the long line is to look for 1 open seat at the counter, have 1 person sit there and order for the group, we done this a few times to avoid the long wait.
Philly is a fun city with lots to do, hope you enjoy yourself there.