Grutas de Lanquín is a large cave system on the edge of town, with a river running through part of it and a visitor route into the first chambers. The cave is damp, uneven, and more atmospheric than comfortable, which is exactly why it is memorable.
Come near dusk if you can. Bats leave the cave mouth in a dark stream over the river, and the sight is better than a routine cave walk. Bring shoes with grip and a torch even if you expect lights inside.