How far inland does the East Coast extend in your estimation?
Around my area: the city of Burlington, VT runs a pretty decent campground at North Beach. This is a popular swimming beach on Lake Champlain, and a nice long bike trail runs through it. This bike trail goes the short distance to downtown Burlington, and up towards the Lake Champlain islands, which are not bad biking either. There are some fine attractions and hikes in the general area. The Shelburne museum is fantastic and eclectic. I like hiking Camel's Hump, which has a spectacular panoramic view from the top on a clear day, but it might be a longer and more strenuous hike than you'd be interested in. Many other hikes (of varying difficulties) are in the area.
Lake Champlain isn't precisely the east coast; but depending on what route you're taking, it may not be far out of the way.
Knoebels amusement park and HersheyPark (both in PA) have campgrounds associated with them and are very fun amusement parks. Again, neither are precisely coastal.
