Morning: Visit to Cenote Dos Ojos. We loved it! Don’t worry if you haven’t hired a car (we didn’t). There are plenty of taxis in Tulum town. We didn’t have trouble finding one around the cenotes either. Prices are reasonable (for a tourist destination).
Afternoon: Lunch and relax at a beach club. The one we went was nice enough but I would not recommend it to you. Tulum beach is full of truly unique beach clubs hence I feel that we could have done better! Most of them are part of a hotel/resort and they will provide you with sunbeds and shade if you have lunch at their restaurant or purchase a day pass.
Evening: Dinner and drinks in Encanto Cantina in Tulum town. We really enjoyed the ambience and the food; huge portions and incredible value!