Definitely take a day and visit Guachimontones.
The pyramids are interesting and the whole site is charmingly uncrowded and only just starting to be made into a tourist destination.
We took a bus from the bus station at Zapotan - probably 1.5 hours or so, but comfortable, safe and cheap. Then it's a walk through the town and up the hill to the site itself. If you are reasonably fit it is not at all difficult - probably a good 40 minutes in total but we broke it up with a stop at the cultural centre in town and watched an informative, but slightly strange video about the site in a largish auditorium where we were the only patrons. The video was in english but clearly translated from the spanish and delivered by what sounded like a computer generated voice. Nevertheless, worth watching.
And the site is lovely - only partly restored but great views.
Thoroughly enjoyed this day out.