Bagpipes buried in Carl's junk folder that he can technically play now, and refuses to.
When Carl explained that Pied Piper merely allows him to play musical instruments, Princess Donut gasped at the implication: he could play his bagpipes now. She immediately proposed starting a band and playing a song at Louis's Second Wedding after they save him. Carl shut it down — the bagpipes are in the Probably-Never-Going-to-Need-This-Stuff Folder, and that's where they're going to stay.
A set of bagpipes stashed away in an inventory folder.
Comic proof that Pied Piper is a musical skill, and Donut's proposed band instrument.