Lac Tritriva is a crater lake in the highlands west of Antsirabe, reached by road and then a short walk. The water sits deep inside a volcanic bowl, making it feel more dramatic and enclosed than nearby Lac Andraikiba.
It is an easy half-day excursion from town if you have a car, driver, or bicycle legs strong enough for the rougher approach roads. Go for the volcanic scenery and the quiet rather than for facilities.