This is my key companion for the past two years, a simple and basic victorinox swiss army knife. You can use them to open wine or beer bottle or cut cake (just like what happened to Kaitlin's birthday cake).

And you can use it a double blade weapon. Just fully extend both the shorter and longer blade on the opposide side of the set and grip it tightly, making your hand a human double axe. This style of weapon is designed more to cut than for stabbing. If you ever get into trouble of being choked, you can use this knife to slash the arteries in your opponent's neck or wrist. But those are for desperate times. Otherwise, go for the ear. It won't kill him but it will make a lot of blood pouring out and that's enough to scare anybody away. I learned this the hard way. There's a 2 inches cut on my right ear.

Or you can use the smaller sharp pieces that extend to middle of the knife. Just hold the knife horizontally in your palm and put the potruding piece between your middle and index fingers. Hold it tight. That will give your punch a little kick.