What we came out with was this: a spider with eight LED eyes that flash on and off depending on the position of a couple of tilt sensors. It's perfect for looking innocuous, and then surprising passersby who brush by it, only to have it flash on at them.
We hung it above our dorm room door to scare visitors.
Step 1: Gather materials
-2 Tilt sensors (pictured below)
-a hot glue gun
-solder and soldering iron
-a ping pong ball
-some kind of cord
-batteries for your LEDs & a battery pack