Gili Air is the closest of the three islands to Lombok and the one most visited by locals from Mataram. It has a genuine village community in the north — the island has been inhabited far longer than its neighbours — and the overall atmosphere is calmer than Trawangan without being as empty as Meno. There are good restaurants, yoga studios, and enough bars for a sociable evening without the noise carrying on until 4am.
The snorkelling off the north coast is reliable and unhurried — smaller groups, fewer boats, and good visibility for much of the year. The island is flat and easy to circle by foot in an hour and a half. Several excellent dive operators run trips to the sites around all three islands from here.