The traditional approach to criminal weaponry is "keeping it simple." Guns, knives, a rabid weasel tied to a stick ... anything will do as long as it conveys the message that you are not to be messed with. If you don't own a gun or a knife, any number of objects can be used in their place. We'd just recommend that they be menacing in some way. For instance, a South Carolina man tried to steal a laptop while making stabbing motions at store employees with a regular toothbrush. In 2007, one of the biggest nerds in the history of crime held-up a food mart with an old NES Zapper.
Didn't they notice that it wasn't plugged in?
There also exists, however, a group that rebels against the orthodox school of criminal thought. Not wanting the world of robbery to go stale, they came up with such innovations like threatening people at snakepoint, where real life snakes are used to intimidate the victims in lieu of broken beer bottles. But the undisputed king of indie-crime (less commercial and more original than mainstream crime), has to be the Colorado Springs mugger who, in 2009, successfully robbed two convenience stores using a Star Trek bat'leth replica.
Pictured: A bat'leth with its typical user
The Secretly Brilliant:
There is an undeniable streak of genius in employing snakes or Star Trek props in your crimes, because sufficiently weird weapons like that will keep the victims from taking a good look at your face, making future identification very difficult indeed. The Colorado Star Trek robber was never caught, and snake stick-ups are actually way more common than you might think. Walking around with an unregistered weapon in your waistband makes you a robber. Walking around with a snake around your neck makes you that weird snake guy that even the cops would probably prefer keeps to himself.