Basically it is all about the coral reefs. You can go Scuba diving, snorkelling, fishing, windsurfing etc. It varies a bit from resort to resort.
Be prepared to pay money though, cost us the best part of £6000 for the 2 weeks (all inclusive).
The reefs are so beautiful you can literally snorkel all day long and not feel bored. The resort we were on had ~100 huts, so it feels like you have the place to yourself. You could walk around the island in 15-20 minutes.
Beware of the cheaper resorts though, as some can be pretty rundown.
Almost forgot, food is the other thing you can do, lots of very nice and well varied food. Cocktails at the beach bar as well.