Tulum has some of Mexico's best ruins, perched on a cliff in the archaeological zone. The remains of a walled Mayan city that once served as a port, the three main sites are El Castillo, the Temple of the Frescoes, and the Temple of the Descending God. There are lots of stairs to climb, small entrances for kids to funnel through, and beautiful ocean views. Kids will love spotting the iguanas. For a treat, have the terrific coconut ices sold outside the entrance gate.