Pre-ozone hole, it was possible to purchase a small Freon horn, not unlike what people used on boats. I got one of these as a kid, and had an absolutely delightful time with it until my mother confiscated it.
I recently bought one of these, but haven't tried it out yet. It seems a bit less obtrusive than a conventional bike bell. You can turn it off when you don't need it so it doesn't drive you nuts:
