My sister surprised her kids last year by taking them out of school early one day, sitting them down on the sofa, and saying that Daddy had something important to tell them. They reacted appropriately...jumping up and down, hugging each other, screaming, etc. BUT the downside is that everytime they have something important to tell the kids, they assume its another surprise trip to DW. For example, when being told they were moving to a different state, they thought DW trip. Big let down and makes the real news even harder to take.
We are surprising our kids in January...getting in the car and saying we are going to their Uncle's house. When the ride gets long, we'll make up excuses until someone recognizes where we are.