The least cost intensive way from Bangkok to bungalow resorts, such as HaadYao High Life many times fully booked during Christmas and new year. Perhaps a good tip would be to reserve a place in a resortbefore you arrive.
If it's boring for you to laying at the beachestry out something different. The island offers many attractions like kite-surfing, ATV Rentals, Elephant Safaris, Djungle Trekking, Diving, Muay Thai, Yoga,a Chinese Temple and Thai Cooking Lessons.
The place is approximately two and half hours away from Don Sak and about forty five minutes from Samuiby boat.
If you are looking for a modern styled hotel while you are staying on Koh Phangan, I would advise to have a look at See Through HaadYao .
Kohpangan's climate is during all seasons beautiful. Nice weather days and a nice ocean breeze nearly everyday. The best time for a visit, late December to late of March could be the best holiday resorts like the Sanctuary and Ananda, offering and colonic fasting.