You will almost certainly enjoy just flopping out on the beach - which hotel are you going to? Taking kids? What sort of thing do you like doing?
There is loads of good zip lining and quad biking, watersports. Definitely worth seeing some of the more inland areas - Black River National Park, and you can do hiking trails there, but it takes a little working out - I can offer some advice on that if you like.
The temple at Grand Bassin is worth a look, a day trip to Isle aux Cerfs, perhaps, Blue Bay for snorkelling and a glass bottom boat trip - it is a marine reserve and there are a few turtles in the bay.
The public beaches are fun - Flic en Flac, or Mont Choisy in the North, and there are bans selling fantastic food for very little money: small mild samosas, rotis and 'dholl puri' -rotis stuffed with chick peas and served with veg curry. It is safe and hygienic to eat from these vans and stalls.
Pamplemousses, the botanical gardens, is lovely.
Casela is good - the restaurant is good and has ovely views, but if you care at all about animal welfare, avoid the 'Walking with Lions'. There is also campaigning about the dolphin trips from Tamarin Bay, especially the 'swimming with dolphins' as the creatures are chased and harassed by boats as they come into the bay to feed.