Central Valley:
Market days at Tlacolula(Sundays I think) and/or Ocotlan are amazing. Both are aimed more for local people in the central valley than tourists, but there are crafts as well. At the Tlacolula don’t miss the nieves(flavored ices) in dozens flavors and if you’re brave borrego and cabrito(sheep and goat) stews and barbecue. Ocotlan food stands are especially good for atole(a hot chocolate cornmeal drink). Ocotlan also has a famous church, small art museum, and the famous Aguilar sisters make their ceramic figurines.
Zaachila has a big former mission that’s interesting if you have more time.
Campers and hikers should check out the Sierra Norte.