I pretty much carry my sketchbook like a security blanket, so I always have a bag or purse at least big enough to carry it. It's usually accompanied by the obvious drawing utensils, erasers, a craft knife or two, maybe some makeup, etc.
At minimum I keep a lil' victorinox rambler on my wallet keychain, and I also like to keep a small roll of tape, sewing kit, and safety pins whenever I can (though unless I'm wearing my biggest-pocketed pants I usually have to borrow some of the boyfriend's pocket space). There are a lot of things that the urban survivalists will tell you to carry in case of disaster, but for the everyday emergency handyman I've found this set works brilliantly, and I always feel a little nervous when I'm too dressed up to keep it basically attached to my person.
I keep thinking I should get a good flashlight to round out the set. That's one thing I usually do end up borrowing from time to time, I have a bunch of cheap LED freebies (the kind of thing businesses give away like pens), but they're not ideal. I haven't yet put much time into researching good ones. :^
As for fun non-necessities to hang on your wallet, my boyfriend has a habit of just hacking keyrings onto random junk and attaching as much crap as possible to his wallet. I think the largest and strangest is a barbie head with the worst hair day you've ever seen.