Whats the best way to spend December in Thailand?

My girlfriend and I are travelling to Bangkok on 7th december for a month. We will spend 2 days in Bangkok, then head up north for 5 days (allowing a day either side fortravelling). After then we have allocated a day to travel to the southern islands. We were thinking of spending 2 days in phuket, 2 days in phi phi then over to ko phangang for 10 days of partys and also some serious chilling! With a day in between each for travelling. Do this sound like a do-able plan to an experienced Thailand traveller? And is there anything we should definately do that we are missing out on?