The Puerta del Diablo — the Devil's Gate — is a freestanding red sandstone arch a short walk west of Tupiza. The shape is striking enough that it features on most local tour-agency posters, and the wider hillside is honeycombed with smaller arches and balanced rocks.
The walk up takes about twenty minutes from the edge of town. Sunset light is the obvious time to come, but you'll share it with everyone else in town who has the same idea.