I feel like I am bragging, but this is funny and helpful: A fellow instructor in my fighting art and I once went to lunch at a Vietnamese place to get some high-energy food. After we exited, we had had to go to our car in the alley behind the restaurant. Some asian thugs jumped us for who-knows-what, and we used what was in our hands for defense: our chopsticks! Ever since then, I have often been known to carry a pair of chopsticks when I can carry nothing else, (and I eat with them too!). It is amazing the kind of damage you can do with a long, strong, pointy-thingy.
Other good covert items that I actually carry:
Steel toe boots
Climber's carabiner (get the right size to fit your hand, works as a very light pair of "brass knucks" and clips anywhere)
Grip exerciser: works like an impact Kerambit, and you can also work your grip!
Large Hankerchief: you'd be surprised
Streamlight Super Scorpian light (good medium size for regular carry, but enough to whollop someone with, as well as blind with 16,000cp)
Leatherman tool (like a roll of quarters but much more handy, and many people get very suspicious when you carry a roll of coins as opposed to a Leatherman)
Stout mechanical pencil or pen
Well, the list goes on, but I make a hobby of this. Those are some off the top of my head that look innocent. Just be creative. Most are just strike-enhancers, but the better ones do even more than that (like chopsticks!). I won't go into details, but with proper training, it is not hard to dispatch of someone with some of these tools.
Of course, I usually carry more effective stuff, but those are some real benign looking items for those times when you are under severe scrutiny.
thaddeus