Door County is nice! Another idea would be to follow the shore of Lake Michigan, and then head west over to Bayfield and the Apostle Islands on Lake Superior. As for The Dells, Devils Lake State Park, which is maybe 15 miles away, is beautiful, the lake is spring-fed and surrounded by bluffs that are great for hiking or rock climbing. There is a nice restaurant, The Ishnala, in Mirror Lake State Park, also very near the Dells. The Dells boat trips are very scenic. The Dells is a tourist Mecca for families, with lots of water parks. If you like to bike, there is the Sparta Elroy bike trail that connects the two towns. It is not close to the Dells, though. If fishing is your thing, Minacqua, or Eagle River are popular.
At Elkhart Lake, are you planning to camp at the track? We have done that several times over the years. It is a great experience!