Banana Gun

Simple yet effective. Take a banana and turn it horizontal. The stem should be nestled in your palm. To use, point at someone and fire with adequate "Bam" noises. Then run away.

Stealth Ninja Fang banana

Scotch (or duct) tape one of those sharp, vicious Staple Extractors (Am I the only one who notices these to be dangerous devices) to one side of a banana. Peel the banana down to the top, so the staple extractor is hidden. (In the picture mine is slightly too high, place it more around the middle) Offer it to an enemy as a "peace treaty". When they take the second bite they're in for a nasty surprise. In office banana battles, this is called "playing dirty".

Piece of Papercut

Take a sharp piece of paper and label it "Urgent". Make it convincing. Leave it on your enemy's desk. When they pick it up they'll be in paper cut city. Be sure they don't know who left the paper.

Banana Battleax

This one is simple. Take a ruler and tape the banana to it. Wait behind a corner. When your enemy arrives, ambush him/her with a swift *thwack* to the head. Or a *sploosh* in their coffee: the perfect way to ruin their day. And their shirt.

Aboriginal Banana Boomerang

(You can decorate yours with elaborate sharpie designs like native banana-slingers!) Take a banana. Fling it.