Grutas de Bustamante is the reason many people make the trip. The cave entrance sits above town in the limestone hills, and guided tours lead through cool chambers of columns, curtains, and mineral shapes that feel especially welcome after the heat outside.
Tours are usually in Spanish, which is fine for the basics even if you miss some of the geology. Wear shoes with grip, bring a light layer for the cave, and try to go early on weekends before Monterrey day-trippers fill the parking area.