I think I owe this idea to Joost Elffers, originally. We've had his calendars on our kitchen bulletin board for years, and somewhere along the line I realized that he had bruised bananas in order to make the spots on giraffes. It hadn't occurred to me that you could bruise a banana on purpose, or for art's sake.
When I put a banana in Griff's snack box, I take a blunt, skinny object (like a closed ball-point pen or the end of a spoon) and "write" on the banana. Sometimes it's a smiley face or heart, sometimes it's a message. I can barely see the impression I've made, but by snack time, it will have darkened into a clearly visible banana-bruise image. I'm not much of a lunchbox-note mom, so this is a nice way of giving him a little love from home while he's at school!