When Bill Watterson interviewed with the L.A. Times about Calvin and Hobbes, he drew this◊ doodle to accompany it (Calvin and Hobbes appear to have no idea who the interviewer is.) More such rare drawings can be found here.
One Peanuts cartoon has Lucy watching the Rose Bowl parade on New Year's Day when Linus enters, asking "Has the Grand Marshal gone by yet?" to which Lucy replies, "Yeah, you just missed him - but he wasn't anyone you ever heard of!" Naturally, the Grand Marshal that year had been Charles Schulz himself.
A borderline example from the 1950s: Snoopy is doing impersonations, and Charlie Brown whispers to Schroeder that his dog is going to be imitating "Mssp Msss." Snoopy then squints his eyes and rolls his dog ears up into mouse ears, looking exactly like...Mickey Mouse. ("Frightening, isn't it?" Charlie asks Schroeder.) What's amusing about this is that Charles Schulz was offered a job at Disney early in his career!