A Few More and One Way to Decide
I totally agree with Michael that the Grand Canyon is too far to try to do in the time and equipment you have. Indeed, I would suggest that you limit your search to places within two day's drive of Philly. That would result in a roughly 50/50 mix of driving and enjoying the area you've driven to. Besides the areas that Michael has suggested you might also want to consider the Great Lakes (think northern Michigan), the Upper Mississippi (Mark Twain, Mormons, Lewis and Clark), or maybe even the Ozarks (Branson, camping, fishing). And be sure to ask your kids what they think. Especially if the adults can't make up their minds, it wouldn't hurt to give the kids two or three options and let them decide - and do some of the research on places to visit once you get there.
AZBuck