I needn't look for a weapon because I always carry a 57cm long (22.5cm Collapsed) telescopic steel baton, which I have developed "quick draw" techniques for and dubbed "Bagunshu" (See Swords forum)
This is great because it is highly surprising, intimidating, long enough to fend off knives, and it lets me exercise self-control (if I choose to). Once drawn there are secondary to finish of the attacker or continue to any other attackers. It is quite important to get a baton of the right length in proportion to one's own arms. The longest standard batons that you can buy without customisation are 77cm long (Fully extended) and feel like a sword. There are also many ways that you can secure them to your body with a "sheath", so you can experiment with different ways to draw. I prefer to carry mine in my pocket, but for the 77cm long it would be too big (Unless you are Eminem with pockets deep enough to hide Osama Bin Laden). I'll write up a nicer article with illustrations one of these days.
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But let's say that for some odd reason I wasn't carrying it with me in a "situation", what would I use... ?
I have to agree on the pens thing. It's quite easy to, in the reverse grip, stab out eyes or puncture "****" along the neck with a pen. I have made cardboard targets to test this. It works great. Dog leashes also work great (Don't ask how I know).
Here, in China, I have used a closed parasol (Quite a nice rice paper one, actually, but still effective) and a closed paper/wooden fan (Have been looking for a steel war fan forever (One where the digits are also knives would be better?), but can't find it. That can also be rapidly deployed.
Chopsticks, TI83+ (And up) calculators, Al Gore documentary, trouser belts, hand bags, olde French bread, your ex-boy/girlfriend, nail clippers, wet towels, cross (to stab), electrical cables with plugs attached, my wife's socks, foreign language, Spice Girls CD, your STDs (Though this won't help you when you need it, it would be NASTY REVENGE!), boomerang (Don't throw it!!!), propaganda, Jehova's Witness' pamphlets, medical malpractise, my Daofu ("Dogi") after training, thumb tacs, parts of fences (preferably pointy), prickly bushes, pot of boiling water, silverware, plates, venomous snakes, poison toads, unusually large cat nip in the pressence of a tiger (Or other vicious feline), (If in water) chum, fire extinguisher, blow torch, fishing rod (Preferably for sharks), posters of teen idols, (When in the pressence of people over 25?) hip-hop, your vehicle, a "friend" with plague-like symptoms (Or ebola), your canine friend, Asian pop music, overly long list of "weapons", et cetera.