Relax on the beach. There are a few hotels I recommend, like Ana y Jose hotel, Coral Hotel, La Zebra, Be Tulum (this is a adult only hotel).
To have dinner or lunch, on the hotel zone is quite expensive, but nice places such as La Popular inside hotel Nomade, or in downtown there´s this amazing place called La Gloria de Don Pepe is spanish cuisine with great food and delicious Paella.
If you are visiting during summer I recommend to go very early in the morning as it gets really hot during the day. Tulum was an ancient Mayan city, that was built next to the beach.
Punta Allen is 2 hour drive from Tulum you can book a tour with your hotel or if you have a car you can drive all the way and rent a boat to see dolphins, turtles, and birds on their wild life. you will get to see the different colors of the ocean it is really beautiful.