Run towards the blackboard carrying a tuning fork...
What it shows:
Waves emitted from a moving source are Doppler shifted to higher frequencies when moving towards the observer, and shifted to lower frequencies when moving away from the observer. In this situation the source is moving away from you, but the raised frequency sound is reflected back interfering and causing beating.
How it works:
All you need is a tuning fork (say 896Hz, see comments), a reflective surface like a blackboard, and plenty of room to take a run at it. A nice fast stride towards the board will create good beats.
The higher the frequency of the tuning fork, the better and more noticeable the beat, so a fork with 800Hz or above should be used. Of course, if you're up to sprinting towards the board, so much the better!