The simplest and arguably the best thing to do in Huacachina. From the western edge of the oasis you can climb directly up the dunes — there is no path, no fence, no fee — and the views from the crest at sunset over the desert and the green lagoon are some of the most memorable in coastal Peru.
The climb is harder than it looks because of the collapsing sand; budget 30 to 45 minutes to reach the higher crests. Take water, take a torch for the descent, and consider taking a thin sheet of plastic if you want to slide back down. The sand cools fast after dark, so a light jumper for the walk back is sensible.