LooseChange said..
Light a fire in the backyard and try to extinguish it, job done. You either manged to put the fire out ... or not, either way the extinguisher is now empty and no longer a pressurised hazardous container. Simples! Don't try to overthink these things.
Such a great idea, on the job experience is the best way to learn.
Maybe set off all your out of date flares to start the fire, 2 birds one extinguisher
There is always this, for those people that look straight through you when you wave.
