Where: In one of the area's cenotes, which are underground pools connected by a network of limestone caverns and rivers. Once regarded as sacred sites by the Mayans, cenotes are accessible for swimming (and getting down on bended knee). The Dzitnup cenote, just outside Valladolid, is one of the best.
Why: The limpid aquamarine pools and their dazzling ceiling reflections will create a wonderfully atmospheric underworld for your proposal.
Where to stay: Chill out at beautiful beach resort Esencia on the peninsula's east-coast Riviera Maya: It's a tropical jungle and Caribbean beach two-for-one, plus it's being close to the ruins of Tulum.