Large set of ruins right on the beach in the great town of Tulum. Just 30 minutes south of Akumal. Snap some photos then work on your tan.
There are many, many cenotes (caves) on the Yucatan Peninsula. Google Cenote Yucatan and check out the photos to pick which you want to see. Rachel went to Dos Ojos and loved it. At cenotes you can swim/snorkel/scuba dive.
There are excellent snorkeling opportunities in the Akumal area.