From previous 'what to do' posts:
IanS: My old helmets go to the local paramedics or St John Ambulance for them to use for training purposes. I want any paramedic removing a helmet in the event of accident to be well practiced, it may well save my life after the helmet I'm wearing has done it's job.
Pepperell: A local MC accessories dealer drills holes in the top, then hangs 'em from the fence. They make wonderful hanging planters... Geraniums, I believe...
They make really cool lamps, too! Go to the Craft Store, and get a kit!
Pictures and how to -- creative ideas.