Swimming, snorkeling, paddleboarding, boating, jet-skiing, hanging out on the beach
The Arenamar Cafe serves food and drinks, including a great piña colada, in the building near the western side of the beach. There are bathrooms there as well.
White Caribbean sand
Lots of people make for more trash, but there are good trashcans located along the beach.
The water at Sun Bay can be gloriously clear, or somewhat milky in the summertime, esp. if it has been stirred up by storms. It's always beautiful though.
Sun Bay is a public park administered by the Puerto Rico Dept. of Natural Resources. It is a huge beautiful expanse of white sand in a large half moon shape. On the left side, it is a rocky shoreline. In the middle is a very long sandy stretch with a small section of rocks and tidal pools near the center. On the right side, there is a long sand bar out to Cayo de Tierra, a small island which is interesting in its own right. That section often has significant amounts of sargassum seaweed.
There is a pavilion (on the right side near the buildings) that was damaged by Hurricane Maria and never repaired. You can get a little shelter there but much of the roof is gone. There are several good spots scattered all along the beach with big coconut palms and other trees to provide shade. Natural shade is the best shade!