There are many good areas. Custer State Park is an amazing campground. http://gfp.sd.gov/st...rectory/custer/ If you have never been to Mt. Rushmore it is pretty special. Avoid the last week of July and first two weeks of August. There is a small event out in the Hills call the Sturgis Motorcycle Rally. Hotels, campgrounds etc are booked up a year in advance. I have never been out there four wheeling, but I know it goes on.
E2A: The 21 year old might like Deadwood. Live table games. Texas Hold Em, Blackjack, Slots, etc