Xetulul is an amusement park in Retalhuleu, with rides for kids and grownups. It has different areas themed like different places and countries, including Guatemala (Mayan, Antigua, Puerto Barrios), Spain, Italy, France and Germany. Entrance is Q100 for adults, Q50 for children, if you do not have and Irtra ID.
There are also many restaurants, gift shop, and arcade, a theater. You can go there easily a couple of days without getting bored.