... If you haven't already booked rooms for the holidays, you should do it now.
The islands in SE Asia are where most travelers want to spend Christmas and celebrate New Year. Koh Phi Phi, Koh Pha Ngan, Koh Samui, Koh Chang - you name it, there's going to be a lot of action on these islands, which means fully booked rooms.
The popular bungalows and guesthouses will get booked first, then comes the less popular alternatives. If you're hoping for a beach-front bungalow, I think it's a little too late too book now... Beach-front bungalows are VERY popular and are booked several months ahead, whether we're talking about beach-front bungalows in Thailand, Malaysia, Indonesia or India.
Here are some personal reviews of guesthouses on Ko Phi Phi, along with other suggestions to where to sleep. And remember; rooms on Phi Phi and other islands in Thailand are more expensive than rooms on mainland. Rooms at say, 800 THB are cheap in 'Phi-Phi'-sense.