We did a surprise trip for our teenagers, but with a different flavor. We told them we were going to spend a week in Washington, DC and do all the DC stuff and maybe 6 flags. We ordered all the stuff for DC and talked it up for months. The kicker is, we had secretly planned a trip to Disney instead. What made it great, is there was a Southwest flight to DC leaving 30 minutes before our flight to Orlando. When they called boarding for DC, they were wondering why we did not get up to claim our boarding spots. It was at that point, we handed them their magic bands and explained that we were on a flight to Orlando and not DC. My son was 15 and my daughter 12 at the time. We hid it good. We did not talk any Disney with them around. We packed for a summer trip, so that was easy enough. We stressed to bring lots of swimsuits and what not. We even secretly asked questions like, "if you could eat at any place at WDW, where would it be" just talking about our last trips. We had a blast. They were so surprised, my daughter cried. It was not like it was our first trip to Disney, but my wife and I hid it from them 100%. Edited to add: It was hard to be sneeky with the Magical Express tags and all that. Fortunately, at the airport leaving, we sent them up to the food court to look for food while we attached our yellow tags to our bags. There were several hurdles we had to leap over to make it a secret, but it was fun to do.
