Cerro Koi is a protected hill on the edge of Aregua, known for unusual columnar sandstone formations that look almost volcanic in shape. The geology is rare enough to make the hill more than a simple viewpoint, and the setting above Lake Ypacarai gives it an added scenic pull.
It is one of the best short nature outings from Aregua. The paths are not long, but the combination of rock columns, scrubby hillside, and lake views makes the visit feel distinct from the town's ceramics shops and lakeside streets.