I agree with seeing the Badlands. While there cross the interstate and take the tour at the Minuteman Missile site. A lot of history and really cool to see. They take you down in the original silo in the elevator to see the control room. You can then see the missiles down the road in their silos. It does not take that long and well worth the time. That is a beautiful area.
If you are in Hill City in the evening eat at the Alpine Inn Hotel Steakhouse. Very reasonable and all they serve is filet mignon. The only choice you have is regular size or large. The deserts are to die for but get there early.
Crazy Horse & The Needles are worth your time. You can see the Needles at the north end of Custer SP.