I recommend the Franklin Institute, Philly Zoo, and definitely a hotel with a pool (Rittenhouse, AKA University City--right by the train station, which is a plus, AND it has full kitchens and suites, so that's awesome--it's only @ 15-20 minutes walk to Rittenhouse Square, maybe the Lowes--haven't stayed there myself). Sounds fun!
Baltimore is also good for a night or two (Aquarium, Science Center, Walters Art Museum for armor, Zoo).
And of course, NYC, if you find an affordable hotel!