I recently got a MakeyMakey. It's a sort of ersatz USB keyboard that can be plugged into anything electrically conductive.
So, I plugged it into my Raspberry Pi, loaded up the Scratch programming environment, and created a fruit and veg interface. See for yourself!
The scripts themselves are very simple.
The MakeyMakey in its default state, acts like a USB keyboard - so it will send up, down, left, right, and the space key. So, it's really easy to use with Scratch's "Key ___ Pressed" sensor.
Download Scratch from MIT (available for Linux, Windows, Mac).
You can buy bananas and spring onions from any reputable green-grocer. Be sure to ask if they're electrically conductive.
Thanks to the OpenRightsGroup for sending me the MakeyMakey.