We've done a few things like this with our kids...
They love the movie Willie Wonka (the original) and were constantly pretending to find golden tickets in all of their food wrappers, so one year they got "golden tickets" made by my husband inside of Disney Beach towels. That was a really fun one. The kept trying to give the tickets to the plane reservation people... the ticket people at WDW, the hotel people. It was pretty funny. They were about 6...
One year, we told them we were just taking them on an adventure, they'd have to see where they were going...That was a really fun trip, because our oldest was only 4 and we had to get up when it was still dark to catch the plane. We put on our adventure theme music (The Lion Sleeps Tonight) and he came out of his room and said, "we can't go on an adventure now, it is dark outside!" The problem with that trip was that they weren't familiar enough with Disney that I don't think they really figured it out until they saw Mickey. Let me assure you that THAT's no longer a problem!
We've also done scavenger hunts that take them around the town in which we live. One clue said they'd be having lunch in the city that was a mile-high...(our layover stop). After the layover, the flight attendant delivered their next clue which was about their final destination. That was a really fun one, too, because the people around our seats got just as excited as the kids once they figured out what was going on... another fun thing was that it was over spring break and I'd convinced them that they'd be cleaning out the basement all week. They were NOT looking forward to that!
This trip they do know about, sometimes the fun is in the planning...although they don't know everything about the trip...

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