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.
