Black Canyon of the Gunnison National Park: Sort of a mini Grand Canyon. Carved by the Gunnison River at the bottom of the canyon.
Colorado National Monument: Reminds me of a small scale of the sites you'd see in Utah's National Parks, like Zion and Bryce Canyon.
Great Sand Dunes National Park: The highest sand dunes in North America.
Mesa Verde National Park: Iconic cliff dwellings of the Pueblo Indians.