Pocket knife is a good thought if OP doesn't have one already.

Lots of threads around on them, and you can easily spend a hundred bucks or more, but I'd lean towards a cheap one. Anything you carry around with you all the time is likely to get lost eventually, and you don't want it to be totally irreplaceable.

Also don't get some freaky huge self-opening combat knife, people will think you're a psycho. And it'll wear holes in your pants. Something like the Ken Onion Chive is useful and won't look totally out of place in business casual clothes.