Antigua is comparable to a "beach town" in which everything is within blocks from another. Check out all the boutiques and handmade goods. Antigua is famous for jade and of course coffee. If you don't want to buy anything you can just say "Gracias, pero solo estoy viendo."
San Martin is the best ~ My favorite restaurant chain in Guatemala, but not at all like a chain that you will think of. Their restaurants are usually in antique buildings with original designs per restaurant, food is wonderful and fresh with great options for more familiar dishes to the typical Guatemalan food but in an elegant way. Plus their coffee is very good.