As Flags mentioned, the entire black hills area is worth seeing and spending time in. Custer Park, Rushmore, Crazyhorse, Horsethief Lake, Sylvan Lake, Deadwood, Spearfish, Sturgis, All of this stuff is worth spending time in if you have it.
Badlands NP, Boulder Canyon between Sturgis and Deadwood,
Devils tower is a must see IMO.
Just depends how much time you have. If You only had a day or two, I'd hit Devils tower, then head towards the black hills and see Rushmore and drive highway 385 through the black hills.
Good luck on your hunt.
-Jake