Great Sand Dunes National Park
One of Colorado's lesser-visited parks, Great Sand Dunes National Park has 30 square miles of dunes including one that is 750 feet tall (228 metres).
You can hike up the towering dunes (it is a five hour round trip to the summit), or even better, ride down them on a sleigh, even in the summer. Also, the park is a great place to take in the Milky Way stretching across the night sky, with the park open all day and night.