One of the best things to do is take a drift snorkel along the southwest coast of the island.
Head out pretty much in front of where the remains of the pier for the abandoned Bounty Hotel are and drift to the left until you end up behind the surf break — don't go too far though as you'll end up in Lombok.
Snorkelling is also good off the coast from Meno Biru, while the area between Gili Meno and Gili Air, more of less out from the village, is good for turtle spotting. We were also told it is good off the northern tip around Adang Adang but didn't try it ourselves.
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