There are three islands, Gili Air, Gili Meno and Gili Trawangan. They are all about 8 minutes away from each other by speed boat. My family has just spent four days going in between these islands but our accommodation was on Gili Air. They're a great little getaway for families, young kids, honeymooners, and even the elderly and really safe for teenagers like me to roam at all hours.
From Bali they're about 1/12 hrs away by a fast boat. Depending on the weather and which harbour you choose to go from. We went from Padang Bai but you can also go from Serangan. Serangan Island however will take around 1 hr longer. If you go from Padang Bai which is an hours drive from Sanur, and there's a really great restaurant that you can get good coffee and food at. It's part of a dive school & is right across from the jetty, called Colonial. There are a number of fast boats that can take you to the Gilis but my family chose Blue Water Express. They have a reputation of being the safest & the ride was smooth & fast. The cost is around $ 100 per person return.
While staying at the Gilis there are a number of things you can do. Gili Trawangan is more of the party island with parties every single night. The hotels take it in turns to have a party. Gili Meno is very undeveloped but it has an interesting lake in the middle of the island which is worth a look, and a well known eco resort. It's got amazing snorkelling around it and has a nice little warung right on the beach where the boats come in. Finally there is Gili Air and that's where we stayed, at the Smugglers hotel. Dad did not like it at first because it was a few minutes bicycle ride away from the beach. But it probably had the nicest rooms on Gili Air and it's associated with Scallywags restaurant down by the beach about 5 minutes bike ride from the hotel. We enjoyed our stay there, ate amazing food including fresh sea food every night at the Scallywags BBQ's and my mum's personal favourite "home made chocolate brownie" for dessert. But if we go again I'd like to check out the 7 Seas resort because they were in the best location right on the beach.
The islands are all very small, the biggest being the Gili Trawangan. To get around the islands you can either walk, bike ride or get the horse carriages. I would not recommend bike riding, as that's what my family did and we regretted it very quickly. It started of very nicely but about 10 minutes into the ride we were having to walk our bikes through soft sand in the blazing heat. So I definitely recommend getting the horse carriage, it costs about $ 5 to go around the whole island and it's lots of fun. But the bikes are great to get around the main areas where the sand is packed hard on the road. If you do ride it will take you about an hour to get around the smaller islands, the same time it will take you if you walk (do not ride).
My favourite thing to do at the Gilis was snorkelling. And I enjoyed hiring a private speed boat to take us around for a whole day. It was extremely cheap and the locals are good with negotiating prices. Our boat driver took us to some great snorkelling places. Another option is to go with him to a half-day tour to snorkel with turtles and look at different sites. It costs just $ 10 and you go to Turtle Bay and see turtles. Then followed by a lovely snorkel just before they take you to the cafe on Gill Meno for a nice lunch. Then one more snorkelling trip before returning to your island. Another great thing to do is to have dinner at one of the cafes on the beach while watching the sun set. The Gilis give you a beautiful look of the sun setting over the ocean and there are many spots, bars & restaurants that you can do this from.
Overall the Gilis are a great little paradise getaway for everyone. They're safe, fun and relaxing and I highly recommend going if you're ever in Bali. One last thing, they are only a few minutes boat ride from Lombok so it may be worth adding a few days on to your trip & seeing Lombok too.