Our deputy head used to do a special assembly called 'The dangers of smoking and drinking.' Many small explosions, and chemical reactions - I think there was an iodene clock involved at one point - and culminating in a large H2 filled ballon being lit by a rather scared Head of Year - shook the dust of the theatre lights. We had the advantage of raked seating though, so all could see even small things. Lots of lights turning on and off too. Large scale physics or chemistry lends itself well to assemblies. You could get a pendulum and challenge kids to sit there as you release it from level with their nose and stay until it comes back. It won't hit them, but even if you tell them they won't stay still! A race which is clearly unfair - give one an umbrella to run with, one two, one an open blanket and see who wins?