Self Defense: An effective self defense tool that can be taken anywhere is your keyring. Simply equip it with one of those 16" (or so) lanyards (the Adidas one springs quickly to mind). Or, if you're not impressed by commercial offerings, make one of your own from 550 cord; this will do double-duty as a small emergency stash of cordage. I tried to post a pic of mine but I'm having trouble getting pics on.

With a little practice this can be swung at an assailant with great effect, especially if you pack a hefty amount of keys. If the assailant manages to grab it you can pull him in close for follow-on techniques, and if you can get it around his neck... Anything outside basic strikes requires a modicum of skill, but nothing too advanced.

This is extremely "low-vis", and no one will give it a second look, even under the most stringent security standards.