We've done this once before and it was really fun to see them get excited (although that time we said we were going to dinner in the city--about 40 minutes away and listened to DD complain about how she didn't want to go. I said "Where would you like to go instead?" and predictably, she said "Disney". We timed the question so that she would answer just as we got to the airport exit. Dh said, "Well, ok....just this once and turned off toward the airport." DD couldn't believe we were serious until we were actually on the plane.
She still talks about it...