Probably the funnest thing that I and my pre-teen cohorts discovered was that you can rip a page out of "Popular Science", roll it into an elongated cone, and shoot it like a blowgun out of a piece of electrical conduit, which at the time was readily salvagable from most construction sites in the evening hours. Packing a weight, such as a small pebble or a fishing weight into the small end of the cone greatly improved the missile's trajectory and range.
An alternate technique involved rolling a firecracker into the tip of the cone. Unfortunately, the launching was very tricky, and the range wasn't more than about twenty feet.
So my friends and I would set up post on a building under construction, and armed with a couple of hundred of these harmless paper ddarts would pelt the unsuspecting motorists below. Never saw an accident as a result, though we did hit quite a few on the hood. The cops nevr caught on, in spite of all the flattened darts on the roadway.
I would highly recommend this activity to our youth, as an alternative to some of the more destructive practices happening these days. Thanks, BlueJay