I remember seeing an app once that was used in stadiums and other public venues that could be used to create a distributed and coordinated lightshow with the audience smartphones. How it worked is that the audience would activate an app, put their phones up in the air, and a loudspeaker in the stadium would emit ultrasonics that the phones would pick up. The phones would then turn their screens on or off, and by using different frequencies, you could switch up their colors or selectively turn on only a couple of phones.

Ultrasonics are a really cool and easy way to have short range transmission of data, or even broadcasting with a loudspeaker. The data rate may be low, but it's perfectly fine for stuff like toggles, or transmission of commands.