We love cape cod and stay in the dennisport area as well. Biking, hiking, fishing, kayaks. If late enough,c an drive out onto nauset beach.
Even farther north is bar harbor and Acadia national park which are wonderful, but not beachy.
Cape may is great too, but more galleries and shops than outdoorsy.
Our other favorite is Lake Tahoe, but that is not east coast.
Are you looking for a beach town?