We just recently surprised our kids with a trip to DL. DH and I packed all of their stuff on Sat. while they were with Grandma and my sister's family. We agonized through Sunday since we didn't want them to know we were going and made up some kind of plausible excuse for our 12 year old to go to bed early that night since school had just let out for vacation. We woke them up at 4:30 am with the question of Do you want to go to Disneyland today??? And our DS 12 jumped right up and started getting dressed and then came into his brother's room while we asked them. DS 7 was the funniest of all, though. He sat up and asked us if it was for real or if we were teasing him??

Then after we had them all up - we told them we were flying instead of driving. There was a lot of yelling and jumping around, even at 4:45 in the morning.
I will admit it was hard to keep it a secret and I had trouble sleeping the last week before our trip because I thought I might burst with keeping it a secret. Then I had to worry endlessly that I had everything I needed, etc., etc. I don't know that I will surprise my kids again, but it was totally fun to do it.
My sister went to DL a few days before we did and she piled all of her kids in the car and told them they were going for a drive and they just kept driving. She didn't tell her kids anything until they were in Anaheim and drove right past Disneyland. They kept the secret for 10 or so hours in the car.

And when they told their kids where they were going, they wanted to go right then. (at 10 at night!) I still can't believe they drove all that way and didn't spill the beans.