The world's best spot for cavern diving is located just 75 miles south of Cancún. Bright white beaches and laid-back visitors make Tulum, a small windswept town most known for its Mayan ruins, the best jumping-off point for the massive network of underground caves dotting the Yucatán.
These caves, called cenotes – thick with tree-size stalactites and stalagmites, silvery fish, and bats – were formed after a meteor socked into the Yucatán 65 million years ago, creating vast sinkholes that eventually filled with water and became, to the Mayans, portals to the underworld. The best way to see these geological wonders is with fins and a snorkel. Book an excursion with Edventure Tours, run by a family who will steer you clear of the tour-bus crowds. Be sure to snorkel through Dos Ojos, a pair of adjacent cenotes that look like huge eyes ($75; edventuretours.com.mx).
When you're done with the caves, dry off at Tulum's El Paraiso Beach, a stretch of ultramarine water prized by kite surfers just a $4 taxi drive from the hotel zone. Book a cabana at Zamas, a collection of palapa-roof huts situated between palms right on the water (from $140; zamas.com).
More information: Skip the $85 taxis at Cancún Airport – instead, exit the terminal and hang a sharp right, where you'll find buses that will take you to Tulum via Playa del Carmen for about $12.