I am also working on ways to surprise my kids. I've come up with the following so far ....
1) Go to our favorite pizza place and see if they will make me a Mickey shaped pizza. Tape a note into the lid of the pizza box. Let the kids open the box when we sit down to eat dinner. (From a Disney commercial)
2) A co-worker suggested today that I should get a cake from the store and have them write on it "We're going to
Disneyland" or make it a Mickey shaped cake and write "See ya real soon" on it.
3) I thought about having my oldest daughter (she is 20 and knows we are going) get a big box and put Disney themed balloons inside. I will make a label that looks like it could of been shipped from Anaheim. When I get home with the kids my daughter will say that a package came for the kids. I will let them open it ... balloons pop out with a message saying we are going to Disneyland.
4) We are leaving on a school day during the afternoon. I thought about waking up a little late and freaking out that we are all going to be late for the day and then when everyone is ready let them know we are actually skipping and going to Disneyland. (This one is hard cause we are driving and I have to pack everything without them seeing it)
I've also considered sending them to school for half of the day. Then picking them up ready on our way out of town. I would write Disney or Bust on the side window or something that would get them excited when they came to the car.
5) Pack backpacks full of travel goodies and let the kids open them. At the bottom put a Disney ticket or Gift Card with your travel dates.
It might help if we knew the ages of your kids.
My kids are 20, 14, 12, 8 and 19 months. My oldest daughter knows we are going and my youngest doesn't understand.
I know that the forums have other posts on this. So I am off to look for those. I hope my ideas helped some.