What ever you do try to plan around the Sturgis Biker Rally (Sturgis SD,an hour SSE from Devil's Tower). I went there the week my bro was getting married (in Minnesota, Aug 2001) and didn't know the rally was happening. I drive over on Monday and there were bikes everywhere. Imagine trying to drive a car thru town and the streets are flooded with motorcycles.
On the way there I stopped at Mount Rushmore. I've got pictures but not on my comp.