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On nights with new moon, when the sky is dark, you can see bioluminescence on the beach. Guided kayak tours are available for ~US$50-60 including pickup from the hotel and transportation. Other tours may be available from random operators off the street for much cheaper (M$400).
Cool off in the fresh water cenote. There is a palapa (gazebo), bathroom (M$5 for use) and a stand selling beers, water and small snacks. It is M$25 per person to enter the cenote area.