Three days is a stopover or a long weekend, and the temptation is to stay on the hotel beach. Do one thing each morning instead; the island is small enough that the beach is always twenty minutes away afterwards. Base in the south-east near Blue Bay, ten minutes from the airport.

Snorkel Blue Bay in the morning while the water is glass, lunch and the waterfront in Mahébourg.
The wild coast west of the airport in a loop; back for the beach by four.
Choose: the mountain and the beach at its foot, or the first boat to Île aux Cerfs. Both are an hour from Blue Bay.