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Gili Trawangan is just amazing. It's my favorite place in Indonesia. A small island only 3km long and 2km wide, it has no motor vehicles or scooters and the primary transportation is foot, bike, and horse. And the vibe here is simply amazing. Everyone here recognizes that yes, they are in paradise and it is reflected in the friendliness and atmosphere. Everyone is just so happy! Just meeting new friends everywhere on the beach, in the sea, at the bar, on a walk. It's fantastic. And it's a very international crowd here with people from all over the globe.
A great thing to do when you first arrive is to walk the entire circumference around the island. It will give you a good feel of where things are and where you will want to go to eat, drink, surf, snorkel, etc. Also, renting a bike is super nice. Expect to pay 25,000-50,000 a day, depending on how well you bargain. You suck if you pay 50.
The main snorkeling and beach areas are on the north east section. If you snorkel, the current runs north south, so start from the top. I like the south zone for restaurants, there are a bunch of nice places. Over on the west side is Sunset Paradise bar and Surf Point. Watch the sunset from the bar, surf at the point. There is also an amazing spot with elevated tiki huts in the trees called Gili Trawangan Resort that is super cool. The food, service, and people there are great.
Of the Gili Islands, Trawangan definitely has the best party vibe. The bars rotate nights, and there is not always a party every night... but always a good time to be had somewhere. Also, there are usually full moon parties here, the one I went to was at surf point, but the spot may change from time to time.
One cool thing about the island is for some reason, they believe that cats have sacred energy in harmony with the island, so there are just tons of cute kitties all over. And no dogs.
Otherwise, just discover the amazing restaurants and zones on this island. Its really easy to get stuck here.