Grutas da Moeda is a limestone cave system near Batalha, close enough to combine with the monastery and other central Portugal sights. The guided route leads through chambers of stalactites, stalagmites, and damp mineral forms that give the area a geological counterpoint to its famous architecture.
It is an easy, managed cave visit rather than an expedition, which makes it practical for families and mixed-interest groups. On a hot day, the cool underground air is a bonus.