It's girlfriend-boyfriend sing-a-long night where Shamy each play an instrument of their choice and sing a song together.
Amy plays her harp while Sheldon plays either the recorder, Theremin, bongos or the piano.
A/N: I can only wish for a "girlfriend-boyfriend sing-a-long night" on the show. If found a story with similar plot or based a story from this prompt— I'm shamyshenanigan on IG! I'd love to read the story.