Theodore Roosevelt National Park in western North Dakota is one of the least-visited national parks, but well worth the four-hour detour from Rapid City, SD. First, the drive up is perfectly straight and perfectly desolate; no towns, very few other cars, nothing but farmland and buttes as far as the eye can see. (Do be sure you pee first, as there are not many opportunities for it on that road.) Great driving, though not as scenic as Montana.
The park itself is like an older, rounder, more colorful version of the Badlands, with more greenery. And a resident buffalo herd and a few wild horses, all of which we saw.