If you're driving through South Dakota on or near I-90, you've got to take a detour through the Badlands. Even if you don't have time for Mt. Rushmore, Wall Drug, Reptile Gardens, or any of the other fine tourist traps in the area, MAKE TIME for the Badlands. If you're in a hurry, you don't even have to get out of your car - get off the interstate at the appropriate exit, drive straight through the Badlands, and get back on the interstate.
The change in terrain is so sudden and so complete that you feel like you're in another world. And the place is BIG BIG BIG. Huge expanses of these lifeless white heavily-eroded hills, many with flat tops that still grow grass. Ugh ... that description doesn't come close to doing justice to the Badlands. Just GO there!