How about a lawn mower you can control from the comfort of your lounge chair? Look no further than the Lawnbot4000. In addition to Arduino, you’ll need “a cheap R/C transmitter / receiver pair, and a couple of electric-wheelchair motors from eBay” to build one. Video after the break.
I have used this interface to control anything from basic LED’s to Bipolar stepper motors, mini-robots, lifeless R/C cars from the thrift store, and even a 100lb lawnmower (all with appropriate motor controllers).