The Great Sand Dunes National Park is the largest sand dune field in the United States, but despite this, it's still not up to the standards of the beautiful white sand dunes in White Sands Park in New Mexico. Anyway, it's worth going to the national park in southern Colorado, which is only about 2000 meters away, just behind the rock (after the mid-May snowfall melts). I recommend that you climb the highest dune, which takes just over a half an hour to climb. The only problem you might run into here is the wind, which is very common.
For me, this is one of the magical places I've ever visited: a field of dunes along a ridge that still had some snow in May. It's not easy to climb the dunes to reach the highest point, but the views are spectacular and worth the effort.