If you're looking to relive a little of the tom-foolery of your youth this weekend, it's tough to go wrong with adopting the occupation of every school-age boy: amateur weapon-smith.
We generally start off a post like this with a warning about how you'd better be careful and not put your eye out. Crossbows, no matter how diminutive, are the kind of thing that needs no warning. The full-size version has been banned by Popes, the Magna Carta, and various governments since its inception. Treat a miniature version with equal respect and remember that "It doesn't look dangerous" is the second most probable phrase—right behind "Here, hold my beer."—to precede an ER visit.
That said, who among us didn't fashion medieval siege weapons out of office supplies to while away a school day misspent? You're going to need four pencils, some packing tape, six sturdy rubber bands, a disposable pen, and some lightweight wooden skewers.
Check out the link below to see the step-by-step build guide for the crossbow and if you're particularly enamored by it, check out the book it's excerpted from: Mini Weapons of Mass Destruction: Build Implements of Spitball Warfare.