11 years straight going to wdw in Dec. I fly in socks, jeans, turtleneck, sweatshirt, down vest and windbreaker. Windbreaker and vest come off on the plane. I take capris and a short sleeved shirt in my carryon bag...then I can change when I get to MCO.
That way, I have heavier clothes for the evenings....it can, and frequently does, get very chilly there. I even pack those one size stretchy gloves...and have used them!!
I normally wear capris, or skorts, with a short sleeved shirt during the say. At night I can usually stay with the capris, but need a long sleeved shirt and a sweatshirt, and sometimes the vest or windbreaker.