As far as I'm concerned, Badlands National Park is the true "Gateway to the West". Along I90, there is nothing geographically notable between the Badlands and the Mississippi River Valley. Heading west, after the Badlands, there are the Black Hills, Devil's Tower, the Bighorn Mountains, then Yellowstone, the Rocky Mountains, etc... Heading east from the Badlands, there is endless farm and ranch land.
The Badlands is an area where erosion has created a near step-change in the elevation of the ground. On the North side, the land is flat, grassy, and about 2660 ft above sea level. South of the Badlands, the land is flat, grassy, and about 2460 ft above sea level. The elevation drops about 200 ft in spectacular fashion.
Badlands National Park is an ideal location for landscape and nature photography. It is far from large cities, so it is rarely crowded. The entire park is free to explore; there are no fences or signs telling you to keep out. The main road passes by great vistas, so heavy tripods and cameras don't need to be carried far. The rising and setting sun, unobstructed by neither mountains nor buildings, lights up the rock in orange, pink, and red. The light is always changing, providing new opportunities for great pictures.