The Minnesota State Parks on the North Shore of Lake Superior are great! Three in particular have very nice campgrounds -- Tettegouche, Temperance River (really pretty hiking there if you like to do that...), and Cascade River. They wouldn't be booked up for next summer yet, because you can only book 120 days ahead. So you would have to count out your days and be on the computer to book at the first opportunity. They WILL fill up quickly.
Further down I see you mentioned possibly South Carolina as well. Hunting Island State Park near Beaufort, SC is quite nice. Right on the ocean.