A knife would be a bad idea as has already been said, if you aren't an expert you could easily get killed. A nice staff might protect you, and they make collapsible ones if carrying around a wood staff is too hard. They can be very damaging, but if you don't know how to use one it's much less effective and of course it can be a pain to carry around. Mace would be the best bet if you don't have any time for weapons training because real weapons can't be used effectively by a novice. Even brass knuckles would probably be worthless if you had no martial arts experience (Including things like boxing). Mace is the only really simple thing I can think of, aside from staying in a group with friends and simple things like that.