The private nature reserve Sierra de las Ánimas is just off the Interbalnearia (coastal highway), 25km toward Montevideo from Piriápolis.
Coming from Montevideo by bus, ask the driver to be let off at Km 86 and cross the highway. In cold or rainy weather, send an SMS in advance to verify it’s open.
There are two good hiking trails, each three to four hours round-trip: one leads to the 501m summit (Uruguay’s second-highest), the other to the Cañadón de los Espejos, a series of waterfalls and natural swimming holes that are especially impressive after good rainfall. Other activities include rustic camping (per person UR$50, by prior arrangement only) and mountain biking.