For our trip this past November, I told my 4yr old son that I had to go on a business / working trip and asked if he wanted to go with me. I told him that 'mom' was going and he could go with us if he wanted as well.
If not, then he could stay home with his grandmother. He asked what kind of business was it, and would he have to work too. I replied by saying that I had to go dig ditches in Ohio, and that if he would come and help, then I could get done faster and maybe have some time to do something else fun.
He knew something was up, but we were adamant that we were heading to Ohio (we're from the New Orleans area and he knew from his U.S. map puzzle that Ohio was in the other direction from WDW).
1st indication that he knew something was up was b/c one of his classmates and best friends was following us there with his family. My son asked about why was 'Mr. K' in the car back there and were they going on a vacation too?
I told him that we weren't going on a vacation that this was in fact a work trip and I didn't know what he was talking about concerning anyone being behind us.
So we drove thru the night and stopped in the morning @ Cracker Barrel for breakfast (Ocala I think) and as we pull into our parking spot, the other family walked right in front of our car.
My son got a huge grin on his face, but didn't say anything yet... His buddy hollered 'I'm going to Disney World!!!' at which point Patrick looked at me, then looked at his friend and said he was going with his dad to do some work. He wasn't sad, didn't look angry, nothing... Just was going to help dad... My wife figured we'd had enough fun and so she gave him the choice of going to dig ditches in Ohio, or going to WDW with his buddy... He said "Disney, but can dad come with us too?!"
We told him it was a trick all along just to surprise him with the trip and he was OK with it...
He started running in circles laughing and shouting.
This year we're going to Orlando for 10 nights and he knows we're going to WDW. He does not however know about the 2nd half of the trip. We try to make two trips a year (one to WDW and one to USO), but this year we decided to only make the drive once so we're spending 7 days onsite @ WDW and then when it is time to pack up and go home we're going to 'get lost' and stop @ the Royal Pacific / Universal to 'ask for directions'.
He was there last June and looks at the pictures from the trip occasionally, so he should recognize the hotel and know pretty quickly where we are.