Thailand is a popular tourist destination from December through to Febuary and it can get very over crowded in the popular tourist destinations. Phuket, Pattaya and Bangkok are by far the most popular and busiest places in Thailand during peak season. People often try to avoid these places and go to the next popular places like Koh Samui, Krabi, and Chiang Mai but sometimes even these places can be hard to find accommodation and can also get very crowded.
Price in these destinations also are up due to these places having a lot of tourists who spend big, so it might be better to try and find other destinations which will be cheaper, less crowded and more importantly also have just as good of a time or even better than the usual tourist hot spots.
Below are some of other great places to visit during the peak season in Thailand
Trang, One of the most underated places in Thailand, most tourists who come to Thailand have never even heard about this place, this is one fo the favourite places the local Thai people like to visit and it is very beautiful. If the locals are raving on about it then it has to be good. Trang is a few hours south of Phuket, cheaper, cleaner and a lot more friendly than Phuket.
Khao Sok, A place which is growing quickly as a great destination to visit. A holiday in a rainforest which is even more diverse and older than the amazon rainforest. The wild life and eco tourism in Khao Sok is definitaly the best in Thailand and perhaps Asia. This has been kept a secret for awhile and tourists and expats in Thailand are only now starting to see why Khao Sok is a great this place to visit.
Hua Hin, is about 3 hours drive from Bangkok and is has only started to become popular over the last few years, the main beach is one of the cleanest I have seen in Thailand and usually only has a handful of people on it due to hotels being built right next to it, it makes the access for other people harder. The nightlife is also starting to get bigger with a number of new bars opening every month, it is a great place to go that is not too far from Bangkok.
There are other places in Thailand that are off the beaten track that are emerging, but the above are the best alternatives in peak season for now.