I'm going to 2nd the Fort Wilderness cabins. Since you are going in December, there's no telling if swimming will be an option. Weather in FL can be so fickle. The Fort has a ton of stuff to do:
FREE: swimming, hiking around the resort, playing on the beach, tether-ball, campfire sing-a-long with Chip and Dale M&G, movies under the stars, basketball, volleyball, tennis, watching the fireworks and Electric Water Pageant from the beach
Small fee: bike rental, canoe rental, pole fishing, wagon ride to look at campsite decorations, s'mores kit for the campfire
Bigger fee: Hoop-de-Doo Musical Review dinner show, archery class, horseback riding, Segway tour