Wall Drug! Wall, SD Surreal and so very glorious...and... a bit tacky - but still wonderful! If ever you are near this attraction, be sure to stop. I recommend the buffalo burger and one of their famous cake doughnuts. The rock shop and the apothecary were my favorite shops.
Two-Day Itinerary for South Dakota's Badlands and Rapid City
Rapid City and the Badlands (75 miles apart on Interstate-90) combine for a great list of must-do stops: http://www.midwestliving.com/travel/south-dakota/rapid-city/two-day-itinerary-for-south-dakotas-badlands-and-rapid-city/