Arenal Butterfly Conservatory is a good change of scale from volcano viewpoints and hot springs. Near El Castillo, it focuses on butterflies, frogs, and rainforest habitat, letting you slow down and look closely at the smaller wildlife that often gets missed on faster Arenal itineraries.
The visit works well for families, photographers, and anyone who wants nature without a strenuous hike. Combine it with the lake or El Castillo side of Arenal rather than trying to squeeze it into a rushed La Fortuna day.