[Bullitt]

We camped at an RV site. It was actually nice because it was near bathrooms, level ground, place to park the car, and near the showers. Really all you need is a place to lay your head, you will be out in the park from sunup to sundown.
Badlands is a great spot too, worth at least a drive through. There's also the missile silo national park on the same highway. Very cool stuff, but you need to make a reservation for that way ahead of time because spots are very restricted.