When Dally falls off of his chair in the drive-in movie theater, you can hear Ponyboy's laughter above them all. When the shot changes to Cherry, you can see Ponyboy with a serious face on in the background but you can still hear him laughing.
S.E. Hinton, the author of the book "The Outsiders," has a cameo in the scene where Johnny is in the hospital after the fire and is lying on his stomach. She's the nurse tending to Dally - the one who says "what did you do with your gown?"