There are several companies which offer boat trips in and around the lagoon, generally to see whales and dolphins. We'd heard of one which had a boat with a glass bottom, so we put our names down and took our friends out in that for the day.
We went over coral reefs, we ate local food on a beach called the Prefect's Beach, or plage du préfet, we went to see a sandy island that only appears when the tide is low, swam about a bit... But the best bit was right at the beginning, when we went to see if there were any whales or dolphins in the lagoon. As it happened, there were: four whales, which is pretty exceptional especially at this time of year (and of which the photos are absolutely rubbish, I'm afraid, so no whales today), and three hundred dolphins.