Espejillos is a run of natural rock slides and shallow pools on a tributary of the Piraí, about an hour and a half south-west of the city near Porongo. The water has polished the sandstone smooth enough to sit on and slide down, which is exactly what everybody does.
It is the nearest thing Santa Cruz has to a swimming hole, so it is busy at weekends and quiet midweek. The access road is unpaved and needs clearance in the wet.
It works as a half-day, and it pairs naturally with Jardín de las Delicias if you are making a full day of the foothills — though the two are separate trips in practice, not a loop.