I am right over the bridge in NJ, just 10 miles south of Philly. Ginger's Mom has suggested lots of places to visit but I would like to put in a plug for the Jersey shore. You can be at any number of Jersey beaches within one hour and there is lots to do there for kids (and adults as well).

Sesame Place is actually located in Langhorne, PA which I believe is considered Bucks County, but is easy to get to following Route 95 North from Philadelphia.

http://www.sesameplace.com/sesame/pa...formation.aspx

If you visit over the 4th of July there is a tremendous fireworks display in Philadelphia. There is also the Camden Aquarium (again in NJ) but just over the bridge from PA.