If you like lakes, you might look into the Great Lakes. As Dutchmen Sport said, Wisconsin has some wonderful state parks, a few of which are on Lake Michigan. They are very popular, so you might be a bit late finding a site (reservable 11 months in advance) but try Kohler-Andrae State Park or Point Beach State Forest. Both very nice wooded/partially wooded campgrounds with electric hookups at many sites. Both are just across a dune or two from the beach. Point Beach has some good sightseeing in the area - some ships and boats (a submarine) to tour, and close to Door County for a day trip.
Another good state park is Indiana Dunes SP near Michigan City, Indiana. It's also on Lake Michigan, and has a view from the beach of downtown Chicago. You can take a commuter train from very close to the park into the city for sightseeing if you wish, or just hang out in the campground and on the beach there. Nice park, electric sites for the most part. Again, very popular - reserve early! Next door to a national lakeshore, with trails you can hike, etc.