I think it would be extremely fun for a music or theater theme cruise that ended in a performance by the passengers.
Jazz/classical music...bring your instrument. onboard activities included clinics, and jams with the musicians, and the last night included a concert by the passengers
Theater...turbo show. 7 or 10 day cruise, the passengers crash on putting together a real show (not a talent show, but something like a well-known musical like South Pacific, Oklahoma, etc.). My community troupe did the "7-day musical" once and had an absolute blast.
Then again I'm crazy..and probably the only one that would ever do something like this.