Tulum is the most popular day trip from Playa del Carmen, about 60 km south along the Caribbean coast. The Mayan ruins sit on a cliff directly above the sea — the only significant Mayan site built on the coast — and the setting is genuinely dramatic. The main Castillo pyramid and the Temple of the Frescoes are well preserved, and you can walk down from the ruins to the beach below. Go early in the morning: the site is usually quiet before 9am, and the tour buses from Cancún arrive mid-morning and crowd the ruins quickly. Combine the ruins with a stop at one of the cenotes nearby — Gran Cenote is the most accessible — and lunch in Tulum Pueblo before heading back on the colectivo.