Be sure to get into Custer state park and spend some time there on the roads. Do the Needles Hiway and Iron Mtn road, awesome drives. You can stay at cabins right in Custer state park or you can camp there. We camped there for a week last sumer. There's Wind Cave and Jewel Cave, both are worth seeing. If you like to hike you can hike to the top of Harney Peak, it's the highest point between the Rockies and the Alps, it's a pretty good hike (I think about 3 miles each way) but it's beautiful. In the Northern hills Deadwood is an awesome town and Spearfish canyon is a beautiful drive also. Another great spot to stay is spearfish canyon lodge, even if you don't stay there drive up the road past the lodge and go up to Roughlock falls, very neat area and park. It's a snowmobile trail in the winter.
Have fun!!