If you only get one stop alone in Cancun, make it MUSA Underwater Museum (500+ sculptures, snorkel or dive). Next on our list are Isla Mujeres Ferry (ferries from 3 Cancun piers) and Ruta de los Cenotes (about 20 cenote parks); the other three, down to El Rey Ruins, are below with where to book.
old Caribbean trade port, daily 8-17
Tue-Sun 9-17, last entry 16:30
underground rivers, daily 8:30-22
about 20 cenote parks
ferries from 3 Cancun piers
500+ sculptures, snorkel or dive