With it's clear blue warm water and convivial locals, Akumal is a great place for those that love the sea/beach experience.Although some of the reefs were damaged in a Hurricane, there is still good snorkelling to be had, or scuba and cave diving. There are two Scuba shops in the town.
Head out snorkelling by yourself, or chat to a local who will probably offer to take you out on his glas bottomed boat.
After that, head down to one of the local eateries to chill out with a cold beer.
There is not much shopping to speak of in Acumel, so your time will be well spent soaking up the sun and enjoying the beach life.
Link: Akumal travel guide - Wikitravel