Antigua has possibly the widest range of eating options in Guatemala. All major international cuisines are represented here, as well as plenty of good local and Central American food, at all price points.
On Saturday and Sunday, tables are set up in front of Convento La Merced, serving, among other snacks, chicken salad sandwiches, rellenitos (mashed plantian balls), enchiladas (pastry with spiced meat), tamales and their Guatemalan variant chuchitos laced with hot sauce and pickled cabbage, along with bowls of atol blanco (a corn-based hot beverage). Talk about comfort food!
A quick word about taxes: most formal restaurants don't include VAT and service charges on their menus, but happily add them to the bill. This can add more than 20% to what you might have expected, so be prepared.