Lie on the white sand beach looking at this hemisphere's longest barrier reef or looking at each other.
You can kayak, swim, and snorkel off the Acamaya beach. Or you can catch some excellent scuba diving and windsurfing out of nearby Puerto Morelos.
If you prefer, you might try eating in one of the three excellent restaurants in Puerto Morelos, like eating Mayan pizza in the Pizza Palapa or eating some sea food while looking at the turquoise sea from whence came your dinner. Shirts and shoes are not required.
Don't forget the wonderful Mayan ruins only a short drive away including Tulum and Chitchen Itza. Many eco parks are also a short drive away including the famous Xcaret.
And if you have a little bit more time you can visit the Cobá and Zibanche Ruins that are a little bit farther but that are also worth visiting.