SD Ellsworth Air force base, open to public, on I-90 near Black Hill if you like military. Nice museum and also group tour inside of base, taking you to 战略核弹发射井
Rapid City: Museum of Geology (Free) on the campus of South Dakota School of Mines and Technology, very nice display, I enjoyed, bought a lot of different mineral rocks. They have all world famous diamond replicas, pink, red, green, blue diamonds.
Crazy Horse, the sculpture 4 times larger than Rushmore. Personally I do not like it, one you paid entrance see a un finished sculpture, two the whole complex all is Indians commercial.
Walldrug, You will see a lots large advertisement board along I-90, DO NOT STOP. There is nothing you can see.
By the way when you planed to go? Do not go to Black Hills 1st or 2nd week of August. I made mistake, during that time, there are more than 200,000 motor cycles in this area (Sturgis Rally). Every place you go price raised, more than $100 rent a camp site, and all booked, normally less than $30.