If you search on you tube, you'll see a variety of ideas. I love doing that every once and awhile, its just fun to see all the kids get excited!
When my sister and I were in high school (16 and 17??), my parents told us they had signed up for some conference in California. Somehow never questioning it beyond when it was and what we should wear (such girls!), we never even thought to look online to see if this actual conference was gong on - it sounded soooo boring to us. Apparently at this conference there was some special program for high school kids and one day we might go to a theme park (Sea World or Knotts, disney was never mentioned). It was over Thanksgiving break and my Dsis and I were missing all these fun activities with our friends. Needless to say, we were NOT excited to go.
On the road trip to California, at our ages, we still never noticed where we were going. Now that I've been back, I learned that it wasn't until we exited on Katella that our parents spilled the beans. My dad pulled over to this 7-11 and pulled something out of the arm rest console, "Oh hey, did we mention we're going to Disneyland?" And it was the disney planning package thing with all our info, tickets, discounts, lanyard vouchers. My sister and I were in shock, wide-eyed, and so happy! We both had to ask though, to make sure, "So no conference? YESSS!" It was the afternoon, so after checking into Portifino, we headed to the park. What an awesome surprise. My parents continued to surprise us with a number of things on trips for the next couple vacations to various places. It became a game to them since we had grown up and little things weren't as exciting anymore or something.
So I agree with the everyone, tell them they have to do something they don't want to, or go somewhere they don't like, then when you get to the airport, or off the exit (driving), just do the simple, "Oh, we didn't tell you?" Waiting until the last possible minute to tell them will make it all the better if they think they're going somewhere "boring" or "lame."
Have fun!!!