Escondido Place is a hot-springs complex north of San Miguel de Allende, built around warm pools, domed bathing rooms, and garden paths. It is a change of pace from churches, galleries, and cobbled streets, especially if you have been walking the centre for a few days.
This is a leisure stop rather than a cultural heavyweight, but it clears the World66 bar because the thermal springs are a real local outing. Go on a weekday if you want a quieter soak, and combine it with nearby countryside stops rather than expecting a full-day attraction.