I'm sorry that you haven't gotten any responses! I've never traveled in December, but when I go in November, I typically pack layers. Lots of layers. Days can get pretty warm, but nights can be pretty cool. In my photos from past years, I'm wearing everything from shorts and a tank top to jeans and a sweatshirt.
Edit: On a re-read, you didn't say December in your post. I must have confused it with something else! In that case, my advice is even more relevant, LOL! :-D