I invented the Monkey Couch Guardian, and wrote a how-to so you can build one, too.
Combine an Arduino with a proximity sensor, and make an obnoxious device to discourage cats and other fur-shedding pets from jumping on beds and couches. This project will also introduce you to the SPDT relay, a fundamental component of hobbyist electronics projects.Monkey Couch Guardian: complete instructions
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects