One of our favorite spots in Iowa was Maquoketa Caves State Park. It is a park full of wild caves (which, frankly, was a crazy experience in itself - I can't imagine the kind of liability insurance they must hold with all of the caves!), meaning that only a few have lights, none have guides, and you enter at your own risk. Now, I'm a pretty good map reader, but their maps weren't the best. The paper maps seemed alright, but we didn't print our own and they were out when we arrived. The maps posted throughout the park, however, were not terribly helpful. So if you're heading out, do yourself a favor and print a copy or take a screenshot with you. 
We also found a couple tourist traps that were totally worth the trip. En route to some tasty (albeit very sweet) wineries in Dubuque, we stopped at the location where one of my favorite films was shot. "If you build it, they will come." You guessed it, the Field of Dreams ballfield. Since it was early spring, there wasn't an eerie cornfield to hide the baseball players of old, but we saw it shooting forth from the earth in left field.
And though they do not allow any formal baseball games, there was a little league team setting up to play an informal practice when we were leaving. I ran the bases, took a few pictures, and generally enjoyed myself. The field has a bit of a sordid history, having been divided between two owners for much of its past, but now it is all owned by the same family and they do a pretty nice job keeping it true to the film.
We wrapped up our weekend with some day hiking at Mines of Spain State Park and a visit to the highest point in Illinois, Charles Mound. As you'll note on this wikipedia link, they only allow visitors in the summer on the weekend. Unfortunately, we missed that detail, and while they didn't kick us out, I felt pretty badly about intruding. They are quite lucky and have a lovely lovely space.We also found ourselves at the future birthplace of Captain James T. Kirk, and a bar that celebrates their proximity to the location!
