1989 Book Shows Miyamoto Working on Super Mario Bros. 3

http://www.gameranx.com/features/id/694/article/1989-book-shows-miyamoto-working-on-super-mario-bros-3/

"Pictured is the bag that Mr. Miyamoto always carries with him when he goes walking. He’s used it so much that the color has faded. But inside the bag are many little items that he says will aid him in thinking up fun ideas. What’s in the bag is a secret to everyone."

Psychedelic drugs.

It will probably be one of those secrets that is revealed after Miyamoto dies, or if Nintendo losses business. (Will Mario ever die? THE WORLD MAY NEVER KNOW!)