assateague island and Williamsburg VA

The original poster didn't indicate whether he had reservations for Assateague State Park or the adjacent National Park, but if it's the State Park, the Maryland Senior Pass gives you half-price camping Sunday-Thursday, and is available to out-of-state residents, which is quite unusual. If you're eligible, you can sign up at the park when you check-in, and it'll pay for itself immediately if you're there for a non-weekend stay.