Definitely Mt. Rushmore (be sure to get the view driving around the left end of it -- very impressive) and Crazy Horse. We also enjoyed Bear Country USA, not far from Rapid City.
You're in for a treat. My wife and I have been in every one of the 50 states, many of them numerous times, and in over 35 foreign countries, and believe it or not, our South Dakota trip was one of the most enjoyable trips we've ever taken. We talk about it often. Be sure to take a bag of carrots for the donkey's in Custer SP.