At that age, I often did a custom-made activity book. I used puzzlemaker.com to make mazes. I would print a maze and then glue a picture of my son and a pic of something a WDW at the "end". You can generate several very quickly.
You could also have a grandmother, favorite person, or yourself tape their favorite books on cassetes. They really get a kick out of listening to someone they know reading to them with their headphones. This especially helps if you have more than one young traveler. I would suggest that you use some new books to hold their attention, as well as old favorites.
Are you creative? Then write your own story and make a picture book for them to follow. For example, use a favorite toy and make up a story about their trip to WDW. Paste or draw pictures to go with the story. The story could be something like, "Annie's favorite teddy bear gets on a plane and buckles her seat belt. Do you have a seat belt? (pic of plane) ....... The bear loves to eat hot dogs at Mickey Mouse's Maagic Kingdom (pic of a hotdog) ....... The bear is riding Small World (pic of a boat & Small World)" You can be very creative or very simple.
Basically, I tried to have activities that went with our vacation plans.