I love the idea of a giant box under the tree for each: when they open it up, a Mickey Mouse balloon floats out, with an invitation to WDW from Mickey attached.
Maybe Santa will bring a Mickey Mouse waffle iron so you can have Mickey waffles for breakfast!!!
As pp mentioned, a scavenger hunt w/mickey gifts at every clue could be a lot of fun too!
Don't forget to order the planning DVD and pick up some guide books so you can daydream as a family about your trip!