Hi Mary,
Just today a friend tweeted about being on a plane when the Captain got on to announce to some children on the plane the surprise: They were headed to their first trip to Walt Disney World! Surprising kids with Disney trips is so much fun, and there are literally hundreds of ideas of how to do it.
Our family favorite was a scavenger hunt around the house where they collected various Disney objects through clues then had to guess what they all meant. However, that is just one idea of many. I hate to "pass the buck," but if you do a Google search for "
surprising kids with disney world trip" you will find pages of other families' experiences with surprising WDW vacations -- certainly more than I can share in this limited space.
I'd also encourage you to visit
here at the Disney Let the Memories Begin website where there are many, many stories of guests sharing their stories of a Disney surprise -- just search for "Disney Surprise Trip." I have to admit how easily I cry happy tears watching those videos.
I hope this helps to move you in the right direction!