Devils Tower National Monument is actually America’s first national monument thanks to Theodore Roosevelt invoking his newly acquired powers under the 1906 Antiquities Act.
Home to the third longest (mapped) cave system in the world, Jewel Cave is definitely a national monument that should be added to your next trip to South Dakota.
Yellowstone is absolutely spectacular with otherworldly thermal works, bison holdouts from a time when they ruled the prairie, a thriving wolf population, and an encapsulation of the great American west.
Spectacular Teton mountains, winding Snake River, gorgeous lakes & valleys, & some of the best wildlife viewing opportunities in the entire national park system.