We surprised our twins with a trip a few years back right after their last week of Kindergarten. We were driving to FL from Indiana so it was quite the feat. When we stopped to get a hotel room after a full day of drivings, one whispered to me "I think Daddy took a wrong turn." It was an unforgettable time and the pictures of them the moment they realized we were at Disney are some of my all time favs. One thing we did was get them each a copy of the official kids guide to WDW to look at as we traveled. It would make a great distraction for on the plane. I intended to hand them the books when they guessed where we were. They are very trusting souls however (we've done other adventures like that too) and didn't catch on until we actually drove under the Welcome to WDW sign.
It was such a precious time. Good luck with your surprise and make sure you've got the camera ready!