This sounds like fun!! Here are some ideas:
* using Disney clip art/fonts and inspiration from Creative DISigns board (they can do anything over there!) create an invitation to WDW and print on parchment type paper and roll as a scroll and put in mail box or mail...you get the idea.
* Use a t-shirt - there are a lot of cool and clever iron on transfer ideas on the Creative DISign Board -- there may be something there that you could use. Maybe an announcement and you put 1/2 on his shirt and 1/2 on her shirt.
* you can order a phone call from a character
* Serve a mickey mouse meal, with food in the shapes of mickey heads (pancakes, mini pizzas, fruit salad) on laminated placemats made w/ the free customized maps, with Disney music playing in the background and pop in a planning DVD after the meal and see if they guess.
* I think the favorite on these boards is a scavenger hunt
* Present them with the kids planning books and proceed to make a countdown chain and home made autograph books/journals.
Make sure the camera/videorecorder is going to get whatever you do down on film to start the magic. Let us know what you end up doing. There also may be some great ideas on the Official Unofficial KOTS (keeper of the secret) thread.
Happy planning.