La Gruta is one of the better-known thermal-spring escapes outside San Miguel de Allende. The draw is warm mineral water in landscaped pools, including the cave-like tunnel and grotto that give the place its name.
It is a commercial spa, but it clears the travel-worthiness test because soaking in hot springs is a classic San Miguel side trip. Go on a weekday if you want calm; weekends can feel more like a family outing than a retreat.