We are more than a year out from out trip but will be surprising DS (he will be a few months shy of 5) with part of it.
We plan to tell him about WDW a week out so he can do a countdown and decide some of the things he'd like to see. The extra surprise will come the morning we check out. We will have a town car pick us up and take us to the port. DS will think we are headed back to the airport. I'm not sure how we will do it, what we'll say, or even when.
Any ideas?
Driving from Disney property to Port Canaveral I have seen a few videos of this. In most of the ones I have seen depending on the age of the children, the parents tell them that they are going to the beach if they want to save the surprise for when they arrive at the port or get close to the port. I saw one video where the parents had a town car and did that and they even had the driver in on it. Him and the dad were talking back and forth about the best place to drop them off for beach access and everything.
I’m thinking of doing something similar next year. My children have a 4 day weekend late September (Thursday and Friday off). I was thinking of extending the break to Monday to take a 3 day on the Dream. Tentative plan is to fly down Wednesday afternoon, do
MNSSHP on Thursday (if there is one) and then board the Dream that Friday (September 25th). If there’s no MNSSHP, I may go a different route and rent a car to visit Sea World on Thursday.
But the story when we leave home will be that after Disney (Sea World) that we are going to the beach for the weekend. I just won't tell them it's a floating Mickey Hotel that is going to the Bahamas.
How I reveal it will depend on how we get to Port. If we take Disney transportation from WDW property, I'll tell them after we check out when we are outside heading to the bus stop. Although my daughter might not believe me because last Disney vacation we had a
DCL bus for Magical Express both ways. If we do Sea World I will have a rental that I will return to MCO before we catch the DCL bus so I will tell them when we are leaving the Sea World hotel. My daughter is a screamer so I won't reveal it in a crowd.
She'll be 12, but my son will be 5 at the time. He calls it the Mickey boat so I'll probably write some rhyme using the words "Mickey Boat" for my daughter to read.
My other tentative plan is to reveal the entire thing before we leave home. My daughter never went Trick or Treating when she was younger (didn't want to) and is now having a little regret about it. So given the Dream has Halloween themed cruises late September if we do MNSSHP I might give her a note that says something like Oneth by Land, Twoeth by Sea. First time Trick or Treating, Let's Do It with Mickey".
at each place setting (from our last trip) and then breaking the news that WE'RE LEAVING FOR DISNEY TONIGHT! They know we are saving up to go, but not that we have set the date. Telling them that morning is the one small consolation I get because I want to just tell them now. DS12 is a major type A planner and I am worried about his reaction. At lease this way he will have 24 hours to process. 
