20. Great Sand Dunes National Park, Colorado

Annual visitors: 442,905

Fun fact: Geologists believe the entire park was once submerged under an enormous lake.

South of the equally majestic Rio Grande National Forest, this one is a must-visit for Star wars fans. The rolling landscape looks exactly like Tatooine, but here, sandboarding and sandsledding are more popular activities than lightsaber battles. The park has the tallest dunes in the country, with the tallest, Star Dune, reaching 750 feet at its summit. The dunes are cradled in the Sangre de Cristo mountain range and offer excellent hiking terrain.