Visit Blue Spring State Park. It's less than an hour up I-4 and you're going during manatee season. Walk the trails and viewing platforms, hear a ranger talk, bring a picnic lunch for the picnic tables & grills.
Take a day to visit Fort Wilderness and other Disney resorts near MK. visit the petting zoo, rent bikes or canoes to wander the property. Take the bus to the marina and ride the boat to the Contemporary. See the Christmas decorations and ride the monorail around to see the decorations at the other MK resorts. Catch a boat back to Wilderness Lodge and do the hunt for Hidden Mickeys (list available at Guest Services there). Walk the back trail back to Fort Wilderness and have the buffet dinner there. If it's late enough, visit the campfire singalong before going back to your car.
Park for free at CityWalk after 6pm and ride the boats to the Universal hotels to see their holiday decorations. End the evening at Margaritaville.
Check out the town of Celebration. They have "snow" falling on Market St. every hour on the hour from 6pm to 9pm nightly during the holiday season, and some street performers on certain nights.
Gaylord Palms has ICE!, an ice sculpture exhibit $$$, indoor ice skating $$$, and it's a lovely place to visit anyway, if you never had - very pretty.