I believe that Great Sand Dunes National Park is one of the most underrated U.S. National Parks. It is uniquely positioned in an area of the country that is known for its mountains, not its sand. It is home to the 750′ tall Star Dune, which is North America’s tallest dune.
I could hike here for hours. Once you get past the first ridge, the crowds dissipate and you may not see another soul for hours. The beauty of this place is that there are no set paths and whatever footprints you leave in the sand will not last long. However, be warned that the temperature can soar to 150 degrees fahrenheit in the summer and the winters are brutally cold. I like to go in April/May when Medano Creek is flowing, the surrounding peaks still have snow on them, and the temperature is normally nice all day.