The gillis are some small islands near Bali, reached by boat(trip is bumpy, just so you know).We only went to gilli meno, the most calm of the three islands.We liked it, but this is nothing compared to karimunjawa or Sumba. But is a Nice quick getaway from the Bali fuzz.Nice snorkling, empty beaches
Snorkelling in Gili T, Gili Meno and Gili Air is a great experience, with luck you can see some turtles. You can buy the snorkelling trip in your hostel or in the streets in front of the beach.