A simple visualization of two types of waves.
The FIFA world cup has been over for a while now, but I still need to share an idea I had watching one of the games when the audience got bored and started doing a wave around the stadion: this would be a great in-class demonstration of how waves do not transport matter! I usually show demos of waves travelling on ropes, but this could be much more fun – to see the shape of the wave travelling when clearly the students are not moving away from their spots.
Depending on how easy it is to calm that particular class down again you might even consider letting them do a longitudinal wave, too.
Have fun and let me know how it goes!