My dad grew up in Sioux City, IA, and to him South Dakota (and Nebraska, both right across the river) was where he and his buddies went to drink. (Iowa had a 21 drinking age even back then but SD and NE were both at 18.)
Actually, I've been to all these places as well. Custer State Park is breathtaking and a fun drive as well. Can't believe no one -- not even sodak -- has mentioned the Corn Palace in Mitchell! Or the Sturgis motorcyle rally.