National parks in the area around DC and through Niagara Falls: Not all that far from DC is Shenandoah National Park, with its Skyline Drive. Then there are also National Historic Parks and National Military Parks all over the East, such as Manassas (Virginia), Chesapeake and Ohio Canal NHP (Maryland), Gettysburg (Pennsylvania), and Delaware Water Gap National Recreation Area (PA).
In Philadelphia itself, there are SO many things to see and do. My husband and I spent hours, two years ago, tooling around the historic district -- Independence Hall, the hall of congress, City Tavern, Liberty Bell. Nearby is Constitution Hall, the Mint, and more. We also walked about 10 blocks down the road to the downtown Macy's store, where they have an ORGAN CONCERT twice a day -- if you like a good concert of classical music, check out this unique setting for a concert. We did the 5:30 concert and avoided driving out of Philadelphia during Rush Hour!
Donna