I love sand dunes. I spent a month road tripping in Namibia and the red dunes were just beautiful. You have no sense of height or distance as there is nothing to compare to. Also was lucky enough to experience the roaring dunes (as the sand "runs" it creates vibration). The highest dune we attempted to climb after an hour and half long hike in midday (45C) in summer was 1200ft. Needless to say, I didn't make it to the top.