The Laberintos de Tlaxco are a set of narrow eroded passages east of the centre, close enough to reach without turning the day into an expedition. The scale is modest, but the shift from plaza and church to dry rock corridors gives Tlaxco a useful bit of landscape.
Wear shoes you do not mind getting dusty. The fun is in wandering the cuts and side passages slowly, not in treating it like a major hike.