If you are not a big fan of the crowds in Cancun, rent a car and drive two hours to Tulum. Close by is the Sian Ka'an Biosphere Reserve where you can get on a boat tour (if that's your thing), walk for miles along the beach or have a delicious meal at Restaurant & Bar Cesiak, the hidden secret with an endless view over the biosphere. You have to pay a small fee to enter the reserve by car but it is well worth it.
Centro Ecológico Sian Ka'an
Fantastic views of the ocean and the biosphere, good food