Right after Wall Drug we visited the Badlands. The Badlands Loop Road (Hwy 240) that we took was about a 45 minute drive. We didn't get out of the car very much here but mostly just admired the scenery. It was a very pretty drive. Many spots made you feel like you were on another planet entirely.
A pamphlet we received after paying to drive through the park said that this is the only national park where you are allowed to hike anywhere you want. Another bit of trivia is that the area was called the Badlands because all cultures considered it to be bad land to travel through.