Luggage and bags:
No overstuffed suitcases, stop packing stuff your not gonna wear, and if your bags are overweight at the check in counter, you'll wind up paying for the extra kilos.
If your planning on doing some island hopping, travel lite, would recommend a backpack for easy on, easy off, when boarding boats.
Clothing/Shoes/Weather Gear: T-Shirts, Shorts, Sandals, maybe a hat, definitely sunglasses, and sunblock, leave your watch at home, your on vacation.
Toiletries and Medical Supplies: Toilet paper, sanitary wipes, and a small bottle of Germ X waterless soap are nice to have around.
Medical supplies, all the prescription drugs your doctor has you on, and or, a bottle of pain killer for the day after a hard nights partying.
Photo Equipment: Don't forget the waterproof / airtight camera cases for underwater photography.
Power adapters, Chargers, Plug adapters and a extra SD Card for those who really enjoy taking lots of photo's.
Camping/Beach/Outdoor Gear: Plenty of snorkel / dive gear to rent, but for the discerning diver, bringing some of your own equipment is cool.
Miscellaneous: The weather turns bad, you have no choice but to stay indoors, a Nintendo DS or other game console can keep your children, or the child in you, entertained.
Luggage and bags:
I've made the mistake before when I took a case around NY and Washington and Canada when i went 3 years ago and that was a BIG no no....so heavy...
Then I went to Thailand for three weeks and took a backpack, packed way too many things and didnt do a test run of walking around the block with it the day before to test for weight and i ended up hauling way too many things around...
Second and third time to Thailand took the same size pack but really only filled it half way leaving room for all the fun clothes and shoes I bought along the way...
MY ADVICE...go for the backpack. Mine is a 70L one. There are way too many bumps on the road in Thailand and if you go island hopping, suitcases are a real pain getting on and off the boat.
Clothing/Shoes/Weather Gear: Definitely need a pair or three of flip flops..they are the only shoes that I ever take unless Im going through bushes and then I'll take some trainers too...but really, even in the evening when people go out for drinks, they just wear flip flops too..maybe some sparkly ones!!!
Toiletries and Medical Supplies: Always carry toilet paper with you. Living in China for the last 5+ years this has become second nature as very few toilets have paper...but it may be new to lots of you. The 7/11 stores sell tissue packs, and loose toilet rolls as they know what you need, so Im often caught carting around a roll or two with me!!