I've a M7 bayonet which I keep razor sharp, used it for years when working with my unit, mostly taking down foliage to cover the forward observation posts. That on the end of my rifle was always a good deterrent.
Along with that a leatherman TTI which was the only tool I needed to take one of my field guns apart (the other needed a mallet too)
For situations where a knife may be considered "inappropriate" I have a spyderco bug, it's very discreet, smaller than a AA battery when folded down, and about the size of the ninja's key when open, but it's the sharpest blade I've ever used (I work in a hospital now, the bug puts scalpels to shame)
I ordered it knowing it was going to be small, but was shocked to see the long side of it's box was the size of the short side of a credit card...