Everyone comes back on stage to play 76 Trombones. The whole cast plays it on kazoos (or whatever the instrument the Tommy character invents). Norm waves at them to stop. After a beat, he blows a whistle.
Low and behold: The University of Maryland marching band literally enters through the house, marches through the audience, and play 76 Trombones with the cast singing along. I think none of the shows I'll see in my life will beat that.