Valle del Arcoiris
Or Rainbow Valley. It features three different parts, all of them interesting. The "Hierbas Buenas" petroglyph site is the first, featuring over a thousand ancient stone carvings from the ancient "atacameño" people. They're from all time periods, from the first caravaners to the Incas. Most tours just visit sites 1 and 2, but that should be enough to afford a pretty nifty panorama of these ancient people's history. The other four sites are of difficult access. Then you've got the village of Matancilla, which is only habited according to season. If you go there in winter, be assured you'll find it deserted. The inhabitants grow a variety of crops, which are easily distinguished all through autumn. Dead wild donkeys are all too common, on the side of the road, while goats and llamas are harder to spot. Finally, you've got the namesake for the place: the ever-stunning Rainbow Valley, a series of hills that display countless colors, from white to black, from blue to red. Usually, all tours visit two of the four roads available, where the best views are to be had: The "cathedrals" and the great valley. Next to the former, to the left side, are some ever-mysterious buildings, made partially of serpentine, which gives them a light green color. They're well-camouflaged, so you'll have to climb to actually see them. On the way back from these "cathedrals", there's a beautiful hill that displays a wealth of colors; if the guide doesn't offer you to walk down the slope, ask for it! It's certainly worthwhile. You can watch a similar spectacle in Argentina, on the "Cerro de Siete Colores", but you'll miss the petroglyphs. It might be worth it to rather book a tour in San Pedro, especially since most tourists are completely oblivious of the Rainbow Valley's existence, so you'll most likely be alone when you visit. Most of the advice given for other excursions applies here.