The best experience I had in San Pedro de Atacama was hiring a bike (US$6 for five hours) and cycling 12km out of town to the surreal Valle de la Luna.
Thanks to the dramatic erosion of salt mountains, this place really does look like the surface of the moon (great for taking silly moonwalking pictures!).
ycle further into the reserve and you'll encounter abandoned salt mines, towering canyons and huge rolling grey sand dunes. The views from the top of some of the sand dunes are just breathtaking - with 360 degree vistas out to chains of perfect cone volcanoes as well as the Andean, Domeyko and Sal mountain ranges.
Near the official entrance to the park is an eroded canyon linked by beautifully-formed caves (the park staff will lend you a torch for exploring if you don't have one yourself).
Morning is the best time to cycle out to the park, to avoid the intense midday heat and the ferocious afternoon winds. Sunset, however, is the most beautiful time to visit, with the light playing technicolour tricks with the jagged rock stratifications and white moonscape (remember to pack a torch if you plan to cycle back in the dark). Most van tours from San Pedro de Atacama visit the park at this time, so you certainly won't have the place to yourself.