walk
The perimeters of all three islands are conducive to a 90 min to 3 hr leisurely walk with no strenuous hills. Just follow the coastline, put on your sunscreen and it makes for a nice morning.
relax
One of the attractions of the islands is the ability to do nothing. With no cars on any of the islands and the lower tempo of Gili Air and Gili Meno, there are several bungalow-style accommodations with verandas that overlook gardens of the ocean. The styles of cafes and restaurants involves small open huts perfect for two to four people to claim for a good part of the day. With a good book and your swimsuit, the day passes quickly when you leave your watch in your bag. The beaches are not exceptional. Gili Meno has the nicest beaches of the three islands. Gili Meno and Air have their best beaches on the west side of the islands. The south side tends to be windy and rough and the north and east have a lot of coral on the beaches, making the sand rough. On Gili Air, the only pleasant beach for swimming is really in front of the Chill-out cafe and Sunrise Resort on the main strip.
snorkeling
You can rent masks and fins off the beach, or contact any of the numerous dive shops to arrange snorkeling at choice spots nearby. A daily snorkeling programme usually involves a 10AM departure and 4PM return, and includes three or four spots around the three islands and a 2 hr lunch break. The cost ranges from Rp 60,000 to 150,000 depending on the shop and your negotiating skills. The quoted price normally includes the mask and snorkel but Rp 10,000 or more may be charged for the fins. Lunch is typically not included.
It's possible to snorkel off some of the beaches, but pay attention to the currents, which can be strong even near the shore. Wear flippers even if you're a strong swimmer, or you'll spend most of your energy fighting the currents.