Though it -was- pretty funny, eventually. DS knew we were planning a trip, but he thought it was next year. He's 4, BTW.
We had an evening flight, so we packed everything up, then picked him up from school. We figured that by telling him a few hours before the flight, he could get some of the excitement out. When we got home and finished dinner, we told him we had a surprise for him, and we said that we were leaving for WDW, tonight! He just looked at us blankly, and said, what's the surprise? We were really disappointed with his reaction, but then we realized that he seemed to be searching the house. It dawned on us that in his mind, a surprise has to be a "thing"; a surprise announcement did not qualify! Oh well, we had a plane to catch ...
He was very subdued on the way to the airport, in the airport, and on the flight, and kept asking if we would take him to the science museum the next day. Finally, when the flight landed and the pilot said, "Welcome to the home of Disneyworld!" the lightbulb suddenly went on. He looked at us with his eyes like saucers, and said, "We're really at Disneyworld?"
He still looked kind of sceptical, until finally we turned down I-drive and he saw the gates.
Suddenly, it was "Mommy, Daddy! This is really Disneyworld!" and he started talking a mile a minute about everything he wanted to ride.
The downside? At that point, it was quarter to one in the morning! We got NO sleep.