Zona Arqueológica de Plazuelas is the best reason to make the trip to Pénjamo. The site was occupied roughly between 600 and 900 CE, and its remains include pyramidal structures, a ball court, terraces, and carved stones scattered through the surrounding landscape.
The small museum helps, but the setting is what makes Plazuelas memorable. It feels less crowded and less packaged than the famous archaeological sites, which is exactly its appeal.