Packing List

Passport (with photocopies)
Travel insurance (with photocopies)
Airline tickets (with photocopies)
Credit or debit card (see personal spending money)
Any entry visas or vaccination certificates required
Alarm clock
Flashlight
Sun hat, Sunblock, Sunglasses
Insect Repellent
Water bottle
Small towel and swim wear
Toiletries (biodegradable)
Sturdy walking shoes/Sport sandals
Money belt
Shorts
Long trousers
Hiking pants/track pants
Shirts/T-shirts
Waterproof clothes for wet season May-Oct. Umbrella or waterproof jacket.
Cover for backpack or plastic bags to keep clothes dry.
Clothes for temples – long pants or sarongs.
Thermarest or sleeping mat and travel pillow for trekking or sea safari trips.
Snorkel gear
Camera

Optional Things
Reading/writing material
Binoculars
Pocketknife
First-aid kit (should contain lip salve, Aspirin, Band Aids, anti-histamine, Imodium or similar tablets for mild cases of diarrhea, re-hydration powder, extra prescription drugs you may be taking)

(source:http://travelhappy.info/thailand/packing-list-for-thailand/)