I've been trying to think of a convenient way to take drinking water with me when I go hiking in a Natl. Park . Suppose I could buy a metal canteen like we had in the army but I dislike the taste and smell of water after it gets warm in a metal container. s/b an exception for common plastic bottled water as the containers get nearly 100% recycled these days due to their value.

Buy a thermal travel mug that has vacuum walls, it will keep your drink at temperature and will never get warm if you've it it in cold in the first place.

On Samui recently, we took our own reusable shopping bags to Tesco and it caused a bit of a stir. It wasn't an issue but the staff seemed really curious we would bring our own bags (we are used to doing this in Hong Kong and the Philippines anyway) and it sparked a conversation between the staff and ourselves. In 7-11 we refuse the plastic bags too - the cashiers here are actually getting used to this and a few times before I could say no, the cashier had already asked 'plastic?'. The more people start doing this, the more it will start Thai people thinking about doing the same.

I was in in Ko Phangan between the 22nd and 29th of December. Here is my experience.

It was not difficult to find accommodation on the spot as many topics claimed. I was able to pick up a fan hut and AC room for 500 BHT till the 25th on booking.com. Both were on Haad Rin sun set beach. After Xmas the cheapest accommodation in Haad Rin jumped to 700 BHT on booking.com. Instead I walked in another place and got an AC room for 700 BHT. I payed day by day; on hostelworld.com the same guesthouse demanded minimum six night reservation and had only expensive beach hut for rent. When I left on the 29th there were two rooms available which were not shown on the web.

In Haad Rin there are also guesthouses, which are walk-in only. Sunset Road is a good address to start. Four guesthouses had plenty of vacancies every day. For New Year/Full Moon the minimum price jumps to 700 BHT. Another location is the road connecting sunrise and sunset beach and there is also a simple guesthouse right on Haad Rin pier.

Some places in Haad Rin rise their prices a lot. For example Coral Bungalows: 500 (before Xmas) - 1300 (Xmas) - New Year 2200. No need to pay this much.

If you are planning to party it is best to stay in Haad Rin. Transportation in Ko Phangan is more expensive than elsewhere in the country; traveling from other beaches cost at least100 BHT one way, after dark and during party nights more. Note: the last guesthouses that are walkable to Haad Rin are Back to Nature, Thaniza and Bird Bungalow. After them starts a hill that is a killer, believe me,

If you ride a bike the best option by far is renting a bike once arrived and start looking for accommodation by bike.

Note: New Year and Full Moon are busier than Christmas.

I also visited Thong Nai Pan Noi beach, often mentioned as an upscale beach of Ko Phangan, and was surprised to learn that after Christmas there were guesthouses renting room/hut for 500 BHT. The best option is the first ”resort” on the left side, Paradise Garden. The village seemed to be a home for roosters but here I didn't hear any screaming.

Chaweng Beach in Samui didn't look good for walk-ins. There are still some few options. In the southern end there is at least one very cheap walk-in guesthouse (near Chaweng Budget) and on the long street behind the main street there is at least one cheap looking place, also in the northern end.

I canceled my plan to stay in Ko Phangan (and Thailand) over New Year as I didn't like ”the atmosphere”. The parties I saw were tired in my opinion. In Phi Phi the nights are more dynamic.

Hostels are not the main form of budget accomm in Thailand , guesthouses and bungalows are . If you are seeking adventure be advised that places like Phuket and Ko Samui (although Westwood has an alternative cove there she likes) and Krabi are quite developed , mainstream tourist destinations and Phuket is fairly expensive.
For more tips see also this - First Timers Thread

I'd try and get direct transport to Butterworth otherwise its a real pain in the butt connecting. There are mini vans from Koh Samui that go via the bus station in Hat Yai. One early morning, the other in the afternoon.
From Butterworth, I wouldn't bother with the train but get the next bus out of town. Buses go to TBS in KL but you can connect thee to MRT into KL.

No. Why book a hotel before doing research?

Samui is a beach and party island really. Loads of westerners go there to lay on the beach by day and drink by night.

Air Asia do flight and boat combos. Personally I would do Bangkok at the start or the end, you can get cheap flights to Chiang Mai from Surat Thani or more expensive ones from Samui. For that matter you can pay a bit more and fly direct to Samui from Bangkok.

#5don't worry about ferry schedules. Go to the desk and buy a ticket to Samui for 400baht. Buses are already waithing outside. You can also buy a van transfer direct to your hotel at the same time for an extra 150 baht.

On the specific islands question. ..depends which ones.The Gulf side....Ko Samui,Ko Tao etc. ..usually has decent weather in June etc.

The rainy season is much stronger that time of year on the Andaman side.

There's plenty to do in Indonesia to fill 8 weeks.

In general, hotels will charge their peak prices from mid-Dec. to mid-Jan. That time frame is fluid and some business will start their peak season prices as early as the start of Dec. through the end of Jan. (especially on the islands). Others will actually shorten it to just the week of Xmas through Jan. 1st, but few do. It is up to the business/hotel. Just about anywhere you go that is well on the tourist trail will be more expensive. This includes Bangkok, Chiang Mai, Pai, Sukhothai, Kanchanaburi and just about any island.

I may be wrong, but isn't Koh Tao in rainy season? I was on Koh Samui the last week of November and it rained quite a lot. But, that was more than 25 years ago. The Andaman side will have better weather during your time frame.

Koh Lanta has an excellent dive shop in Kantiang Bay. You can private message me for the name and contact info. But, they will not be as cheap as some of the Koh Tao places. There are good dive sites near Koh Lipe as well. Just remember that prices for accommodation will be high during your time frame.