It was in Peanuts. I remember having it in a Peanuts collection. Charlie Brown had been seeing baseballs everywhere and had gotten a rash on the back of his head that looked like baseball stitches, prompting him to wear a bag over his head. His doctor sends him to summer camp to get his mind off baseball, where (due to the bag over his head) he becomes the most popular kid there.
Because he’s Charlie Brown, it all goes pop as soon as he takes the bag off.
Hmm, given your sig, I would have though your favorite would be when the Pogo characters dressed up as Little Orphan Annie characters, but found it hard to see with “blunked-out eyeballs”.
There were other tributes, but I don’t remember any actual guest appearances. However Pogo often played with what today we’d call meta humor: characters saying they worked for a comic strip, complimenting the art work, striking matches against the side of the panel, or grabbing a speech balloon and inflating it.
Dick Tracy has had numerous crossovers over the years-Little Orphan Annie, The Spirit, Barney Google, Fearless Fosdick(dream episode, of course), Gasoline Alley, Brenda Star and a sort of Batman crossover involving the Penguin’s brother.
Mark Parisi’s “Off The Mark” strip has lots of crossovers of both comic strips and cartoons, including the many he’s run showing the “Betty Rubble Center For Recovering Toons”. Here’s an example.
Aeire and R. K. Milholland used to do crossovers between Something Positive and Queen of Wands all the time. In fact, when Aeire closed her strip down, R. K. adopted Kestrel into Something Positive. She shows up every once in a while.
Liberty Meadows featured Catherine Guisewite’s Cathy several times, and not on a good-natured ribbing kind of way. IIRC, they asked him to knock it off.
Back when Funky Winkerbean was just starting to become an annoying exercise in self-indulgence, its author used the strip to wrap up old plotlines from his recently-canceled John Darling strip.
When The Phantom got married, Mandrake the Magician and Lothar were guests at the wedding.
Holy crap are those Pastis strips awful. Family Circus is terrible but Pastis isn’t making the case with those strips that he’s better. Pastis only told two good jokes in those 20-ish panels. He reused one of those jokes enough times that even it died of embarrassment.
I liked them. Obviously, humor is subjective, but like Rat said, “Comic strip humor is comic strip humor.”