Excursions, excursions, excursions, parasailing, ziplining, paddleboarding, all terrain vehicles, kayaking, snorkling, scuba diving, horseback riding, hiking in the desert, hiking in the jungle, visits to the ruinas if in your area. That's what I do when I'm there.
And biking, running, swimming, walking around town. If you can't sleep, go back to the room and turn on the TV and pick up some Spanish. It's easy.