Sandboarding. It's a lot like snowboarding from what I've been told, but where you land in sand instead of snow when, an not if, you fall off.
Given my generally poor sense of balance I did the sensible thing and went down hill face first instead - faster, where falling off doesn't carry as much of a risk of injury; it just results in even more sand being wedged in to your body.
Huacachina itself is a tiny oasis just outside Ica, which the Lonely Planet describes as a "gringos' playground" which is a fair summary. It also says that you can "get lost here for days", which I can believe. Warm, sunny, easy going, an endless supply of randoms to chat to and drink with, and lots of my current favourite place - hammocks.
You only really need one night here, as the dune buggy tour + sandboarding only takes a couple of hours. I've been here for 2 and am considering a third, as all I've got to do is a half morning trip to "the poor man's Galapagos" between now and Wednesday, when I'm due in Lima.