It is a perfect idea to bring your hammock to the island, you can rest at anyplace with trees. And since there are plenty of trees on the island, you can rest and relax almost anywhere.
At the weekendmarket in Bangkok you can buy a hammock for only a few dollars. Sometimes you can also find a mosquitonet especially for hammock which will be necessary if you plan to sleep outdoors in your hammock.
Luggage and bags:
Strong and secure luggage or backpack would be a wise choice.
Clothing/Shoes/Weather Gear: Light and comfortable clothing is a must, good shoes for hiking in the jungle would also be good.
Toiletries and Medical Supplies: The usual bathroom gear and maybe even a small medical kit, just in case.
Photo Equipment: A good quality camera to capture the wonderful sites.
Camping/Beach/Outdoor Gear: Sunglasses and good sunscreen