Presa Brockman is the easy nature break from El Oro: a reservoir in the forested hills south of town, used for walks, cycling, and low-key weekend outings. It is not wild country, but the water and pines balance the mining streets nicely.
Go if you have your own transport or are making a full day of El Oro. In winter, some travellers combine the area with the monarch butterfly sanctuaries in the wider region.