Don't pay the injuns to drive into Crazy Horse, it's a filty, trash strewn tourist trap. If you have to see it to check a box for some reason, you can see everything you need to see from the road.
Wind Cave, Jewel Cave, Custer State Park drive, Hot Springs and the Mammoth Site are all good stops. Deadwood/Lead if you get up to the north side and/or rent some UTV/ATVs depending on the amount of time you're planning on being there and do a day ride through the massive trail system in the Hills.