We surprised our girls with a trip over Christmas break a few years ago. They were going to miss the last day of school before Christmas break, so I told them months in advance that I had scheduled a dentist appointment for them that day. I told them since it was a day that they don't do anything at school anyway, we would just take the whole day off school, go to the dentist, then go shopping or something. I wanted them to know they would miss that day so if someone at school said something to them about missing, it would not be a shock to them.
My parents left a few days before us. They were driving. I was able to pack most of our stuff ahead and send with them. They also took our small tree, ornaments and presents with them. They told the girls they were going someplace historical. They "borrowed" our van to go on their "historical" trip in case my mom found a piece of antique furniture she wanted to bring home. My kids are not too interested in going to historical places or antiques, so we knew they would not ask any questions.
The day before we were to leave, DH came home from work and "announced" that he had to go on a trip for work the next day and would need a ride to the airport. This happens a lot, so they did not question him suddenly needing to leave. We knew we would have a little last minute packing that would need to be done and if they came across us packing, they would just thinking it was for dad's trip.
The day of the trip came and we woke early to drop dad off at the airport on our way to "the dentist". I had scheduled one of those Mickey phone calls from the Disney website to call my cell phone when I thought we would almost be at the airport. When the phone rang, I turned on the video camera and told the kids the call was for them. They listened to Mickey tell them that they were going on a trip today. They were confused so we said "We are going on a trip today. Were going to Disney World!" They were in shock and did not believe us. The phone rang again and it was my parents to tell the girls that they were in WDW waiting for them. We reached the airport about 5 minutes later and when they saw their things in the trunk, it started to sink in. I don't think they could totally grasp what had happened until we were at the gate ready to board the plane.
It was so hard to keep the secret for over a year, but it was so worth it! It was so great to see them put all the pieces together. You could see their faces thinking it over, then one of them would say, "That's why Mama and Papa are gone. That's why they took the van. That's why we aren't at school." I would do it again in a heartbeat but they are older now and I'm not sure I could pull it over on them again.