Xcaret is a popular "eco-archeological theme park" in the Riviera Maya area south of Cancun. Plan to spend an entire day, swimming the underground river, snorkeling, touring the butterfly pavilion, aquarium, Mayan Village, Jaguar Island; relaxing in hammocks; eating at great restaurants; maybe sign up for a premium experience (dolphin encounter; snuba; sea trekking), catch some live performances, and definitely don't miss the excellent Night Show.
*photo © Teresa Plowright
