The Museo Lacustre de Atitlán is a small museum near Calle Santander that explains the lake's volcanic formation, ecology, and Maya history. It is compact, but useful if you want context before crossing to the villages or hiking the surrounding slopes.
The museum is a good rainy-hour stop rather than a headline attraction. Pair it with a walk down Calle Santander and the lakefront. Exhibits are modest, but they help turn the spectacular view outside into a landscape with a deeper story.