Nice to see someone asking Philly questions
1) I think one day is absolutely enough time to cover the historic section (Independence Hall, NCC, Betsy Ross house). From the US forum on the Philly airport thread, there's this nugget:
What to do in Philadelphia?
Get a cheese steak. The big three are Pat’s, Geno’s and Jim’s. Pat’s and Geno’s are across the street from each other at 9th and Passyunk. Jim’s is at 3rd and South. None of them are easily reached via regional rail (commuter train...see below). However, the next best thing would be to go to the Reading Terminal Market. A farmers market of sorts, you can definitely find a good cheese steak here…along with a lot of other good foods. Another decent bet is any one of the street vendors. And you’ll get a fair price from them, too.
Historical. The National Constitution Center, Betsy Ross’ house, Ben Franklin’s grave, Independence Hall and the Liberty Bell can all be reached via reached via regional rail (commuter train).
There is also a self-guided walking tour of the historical areas that runs daily during the summer and on select weekend days during the spring and fall. Check out
www.theconstitutional.com for more info.
anything else?
Take the Mural Arts tour. Driving through Philly, you can't help but notice some of the amazing murals on the sides of buildings...the ~whole~ side of the building. There are numerous tours available...Mural Arts Tour.
Or, check out
www.gophila.com for a longer list of cultural events and entertainment.
2) Other stuff to do...how about the Art Museum? You can definitely spend a long day there.
3) Airplane museums? Not that I know of. However, we do have the
American Helicopter Museum in West Chester (plan on an hour drive from either the airport or center city depending on time of day). I've been a couple of times on school trips with my kids and enjoyed it (not sure, though, I would have liked it as an adult on my own)