If you are into old bones, check out the Mammoth Site in Hot Springs. Also, keep in mind that parking for Mt Rushmore is good for a year, so go once during the day, again at night for the lighting ceremony.
Custer State Park is a nice drive, and make sure to drive up Needles Highway. There are also several picturesque lakes, Sylvan Lake and smaller ones. Great for a picnic or just sitting and reading a good book.
Reviews I read of Crazy Horse said the tour is really not worth it unless you are really into native American culture. You can get a good view of the sculpture from the road, and that was enough for me. Others' mileage may vary.
