As 4runnerguy mentioned, Niagara Falls is pretty much as close to midwest as it is to the (northern) east coast. New York is a pretty long state.
My suggestions:
• Letchworth state park
• Knoebel's amusement park in Elysburg, PA, if you want an amusement park. There are others that are also good—Canobie Lake, Lake Compounce, Seabreeze, Kennywood, and HersheyPark are all very nice, if of varying sizes and prices—but I think Knoebel's is perhaps the best and most affordable for families, and having on-site camping (in what is admittedly an otherwise rather forgettable campground) is fantastic.
• If you time things right, seeing some of the Little League World Series in person is fun (and free).
• Mt Washington and the White Mountains in general. Traversing the Kancamagus highway (and stopping at some of the numerous stops) is a lovely way to spend a day.
• Boston does offer a lot to see and do, but not a lot of camping. The aquarium, science museum, children's museum, and some of the historical sites are all top notch. Mass transit is highly recommended for getting around Boston.
• The Shelburne Museum in Vermont is quite a unique and fun place.
• Acadia National Park
If you like hiking, there are a multitude of possibilities, from the long and strenuous (Appalachian Trail and the Long Trail, for example) to brief and easy. Mt Ascutney State Park near Windsor, VT is a nice place for hiking. If you hike from the campground at the base, it's a pretty strenuous 10 mile or so round trip up and down, depending on exactly which trails you use, or there's an auto road that goes nearly to the summit. Around the summit area are several rock ledges with views and a viewing tower which gives a 360° panorama.