Near Deadwood is the Devil's Tower, the outcropping featured at the end of Close Encounters of the Third Kind. Well worth the detour. I also really enjoyed the Jewel Cave, near Mount Rushmore. I went a number of years ago and went on the spelunking tour. You had to certify that you were in shape to do it and had to squeeze through a narrow cinder block to be allowed on that tour, but they had two others, less intense, that seemed pretty cool. The Corn Palace in Mitchell, SD is also worth a peak if you happen to be passing by and it's the right season. Wall Drugs is worth stopping for. Just follow the roadside billboard ads. Add in the Badlands, Deadwood and Mount Rushmore, and you have plenty to see. I really enjoyed my trip through South Dakota. Have fun.
Michael Siskin