Mt. Rushmore is the least interesting thing to see. Crazy Horse is great, lots of history in the museum. And I will also put in a vote for the needles Highway.
The Devil's Tower is really good, but only if you plan to hike a bit around the base, as that gives the best perspective on the structure of it.
The caves in the Black Hills are also great. It was a few years ago that we went, but I think it was Wind Cave? The only bad part was that I bashed my head into the roof climbing out!
, so keep your head down!