4. Great Sand Dunes National Park and Preserve, Colorado
The wind- and water-sculpted silica spires of this park in the San Luis Valley of southern Colorado culminate in the 650-foot High Dune and 750-foot Star Dune, the tallest in North America. Climb the dunes, and you might see a few brave souls skiing or sledding down them. If you have more time, hike to nearby Zapata Falls for a terrific valley view.