The Chichicastenango market is wonderful for many things, not least of all the food. Join the Guatemalans at the long picnic tables and order a portion of fried chicken. Don't be put off by the large vats of oil--that's what makes it so tasty!
Market days are Thursdays and Sundays but I highly recommend getting there the night before to watch vendors set up their stalls and join them in getting freshly made pupusas from the carts in front of the church. Get up early the next morning and hit the market with the locals, before the tour busses arrive from Panajachel and Antigua.