I'm taking my 14th trip to bangkok this December, arriving 2 days after you.
Since you have 2 weeks, Choose the north or the south, but not both. Running around spending days traveling won't make your trip very enjoyable.
In my humble opinion, you should go to Bangkok, stay on Koh San Road for a day and then take the train to Suratthani and a Ferry to Koh Phan Ngan and stay there until it's time to leave. I know there are so many things you want to see, but you will just stress yourself out if you try to see a bunch of stuff in 2 weeks. Take the money you would have spent on extra plane tickets and go scuba diving, elephant riding, jet skiing, fishing, do archery, go out for sushi, and buy drinks for your new found friends at the bar. There is a huge party on Koh Phan Ngan on Friday December 28th for the full moon, that will continue right until New Years Eve. If you don't get to Koh Phan Ngan until the 28th, book ahead. Really. I've seen people sleeping in hammocks in bars on the other side of the island, the night before the full moon party. Koh Phan Ngan is my favorite place in the world. I've spent months there. I've been all over Thailand and for a two week vacation, it's the best.