I think our Disney Tips board may have some suggestions. Are you giving them the surprise ahead of time or when you leave?
We surprised our kids once. We had the car all packed, woke them up early and told them that we were going to Grandma's. At that time it was about a 6 hour drive to Grandma's and 12 hours to WDW. My older child figured out after about 6 hours that we should have been at Grandmas, and besides how come nothing looks familiar? We told her we were taking a new, more senic route! She got really suspicious when she figured out we were in South Carolina! We finally stopped at a rest stop right before the Florida border and I told DH she was going to figure it out as soon as she saw the Welcome to FL signs so we better tell. I had gift bags made up for them with their mouse ears, wallets with spending money, new tshirts, and a few other things. I told them "Well, you are right, we aren't going to Grandma's. Here are some things that you will need where we ARE going."
They loved the surprise.