A sticker from the Ultimate Fun Book depicting Waldo's Key.

Waldo's Key first appeared in The Ultimate Fun Book. Waldo lost his key on every scene and the reader had to find it in each one. It would later appear in following books and be reworked into the special editions. No one, not even Waldo himself, knows what the key unlocks.
Key letter

Wilma asks Waldo what the key unlocks in the Ultimate Fun Book.

Books featuring Waldo's key