I'll just go ahead and second everything that's already been said. Spearfish Canyon and Sturgis Canyon are pretty drives. The Homestake Mine in Lead is a big ass hole in the ground that is worth seeing. Devil's Tower in Hulett, WY is a nice stop as well. Be sure to grab a beer at Crow Peak Brewing in Spearfish.

I lived in the Black Hills for 10 years so I'm pretty biased about it.