Love the surprises! We kept the secret for FIVE MONTHS before our first trip to DL last year. I had some people tell me I was mean for not telling the kids ahead of time, which made me feel bad

But my oldest has...er...issues with anticipation and we just didn't feel up to weeks of her throwing up every night. Every.single.night!
We didn't do anything elaborate. Got them up and ready for school. Then DH sat them down for a stern talking to (I'm sure mommy holding the video cam must have confused them), explaining that as they'd taken soooo long to get ready, we were now late for school & work...so why not just go to
Disneyland instead. The kids squealed with excitement and I shut down the camcorder to start herding them to the car...then my oldest asked her dad 'are you kidding us?' and mean daddy that he is said 'yes'. Oh how I wish I'd left the camcorder running! It was happiness to despair in the space of a nanosecond! We all rushed in to assure her that no, no, no...we were just kidding about the kidding and yes, we really were going to DL!!!
As a follow up, my same child had to have five teeth extracted about a month or so after we got back. She knew she had to have the teeth out but she didn't know I'd already booked the appt -- until she got up that morning and came downstairs. The running joke in our household is that you can never be quite sure what's going to happen when you wake up - could be Disney...could be the dentist!
We have a cruise/DL trip planned for September. We're going to tell them about the cruise, but not till the weekend before. I figure she has to learn how to manage her excitement at some point, right? We won't tell them about DL until we're in the taxi on the way to our hotel ('oh darn, the cabbie missed the exit for the airport! Oh well, might as well go to Disneyland for a couple of days')