Try the Chater Lea 'Road Clearer' whistle! Hung around your neck on a lanyard -- sounds like a mini air raid siren . . . (There's a sound file on their page).
These are great because you can make more noise than with a simple "PING" one. I guess they are heavier than those, though.Now the next questions is, should I get one of those ones with the internal gearing mechanism?
They seem like more fun!