Cenotes
Rent a mask, snorkel, and fins, and drive, taxi, or bike to a few of the area´s cenotes: Cristal, Escondido, San Erik, Calavera, Gran Cenote.

Coba Ruins
Just a quick drive or bus ride away (about 45 minutes drive). Head out as early as possible so you can take advantage of the day, and plan to stay for lunch. 

Muyil Ruins
A short drive south of Tulum down the main highway. Definitely bring your own lunch (ask your hotel if they can do a bag lunch for you) and plenty of water. This archaeological site is not as developed as most.

Xcaret

Xel Há
In addition to the well known snorkeling and nature park, there is also a small archaeological site located just south of the park, on the main highway.  

Yal Ku Lagoon in Akumal
Located at the end of the road that curves around Half Moon Bay in Akumal. This is a great place for snorkeling. Plan to have lunch at one of the restaurants in Akumal.