This fall I've been hard at work collecting fans, switches, PVC pipes, and all sorts of other parts to make a little DIY project that my son is going to flip for this Halloween. Any guesses as to what I'm building? 
Don't worry, I'll post here once it's finished and share the step by step instructions! 
I would have guessed a potato cannon based only on the PVC pipes. But the back pack and the police tape are totally throwing me off. Can’t wait to see!
I too think it is something that blows up or has a projectile, Its thee backpack too that isnt making sense, but then that could be what you are hiding it in?