This 16km round-trip (three hours one way) follows the Valle Río Eléctrico. There are some stream crossings with sturdy tree trunks to cross on and one bridge crossing; all are well-marked. From Hostería El Pilar walk 1km northeast on the main road to hit the signposted trailhead for Piedra del Fraile, near a big iron bridge.
Pass through pastures and branches left up the valley along Río Eléctrico, enclosed by sheer cliffs, before reaching the private Lodge Los Troncos. There's a restaurant but no guest kitchen. Dorm rates are cheapest with your own sleeping bag. Reservations are not possible since there are no phones – simply show up. Visitors must pay a trail fee (AR$300). The campground has a kiosk, a restaurant and excellent services, and the owners can recommend trails.
Buses to Lago del Desierto drop hikers at the Río Eléctrico bridge.