The bicycle coalition I run recently set-up a booth at our daughter's school for their spring fair. Her school's theme is health, wellness, and environmental studies so we thought it would be appropriate to have something fun and environmentally friendly for the kids to do - that would hold their attention while we talked to them about bicycle safety.
We had our local bike shop save old inner tubes for us for a few days (didn't take long to have a TON of tubes). We got about 200 bracelets out of about 2 days worth of inner tubes.
We cut them down into roughly 2.5 inch pieces and used a small hole punch to make a hole in each end like so:
Then thread a scrap of fabric through that's roughly 1.5 inches wide and long enough to wrap around your wrist and tie (15-16 inches at least). Like so:
The rubber stretches enough to feed the fabric through pretty easily, though if you do a whole bunch of them, your fingers get really worn out! It was a great way to keep the kids hanging around the booth and get them talking about riding bikes and wearing helmets and whatnot.
...and now we're totally trend setters ;)