Do you have pictures from a previous WDW trip? One thing we have done that our kids really love (even the older ones) is to turn some previous trip pics into coloring pages. They love coloring their memories and it gets them excited about the trip. This will keep my 5 and 8 year old entertained for a long time. We always buys some special crayons, like the multi colored twist ups and stuff we don't normally have, which helps to keep it new and exciting.
Our kids love treasure hunts so we do them in the car...I make lists of things they would see along the way and they check them off as they go. When they complete one, they get a prize. A small toy or new book, just something small usually from the $1 store just to keep them interested.
My 5 year old loves to have books read to him so I have recorded some of his favorites onto my mp3 player with me or dh reading them and using the 'turn the page at the tone' and we have a small keyboard I used to make the tone. Now he can read along with me in his actual book while listening to the mp3. Helps to have an older child who can set up the mp3, hand them the book and make sure it's playing, etc.
We also make the sayings "how much longer....are we there yet...is it almost time....etc" off limits but we make it into a game. We give the 'manager' a book of small stickers and a score card with everyone's name on it. The manager counts with a sticker every time someone says an off limits phrase. The person with the least stickers gets a very desirable prize at the end of the trip. Last year it was an extra dessert of their choosing (two of my kids will do ANYTHING for dessert
They can try to get around the off limits sayings, but it forces them to be creative and it gets really funny. We laughed a lot at this!
Have them do round table story telling. One kid starts, the next kid adds something, the next kids adds more and so on. You can jump start this by showing them pics from the last trip and they have to base the story off the picture but the story can't be the truth about what's going on in the picture. It must be made up. Again, a great way to keep their minds busy so they don't notice the passing time.