Morse code memorized for whistling, and passed on from one generation to the next. I know, it;s just another form of brainwallet, and a retarded one for that matter.
I think you'd have a problem with capitals letters using morse code. It may not provide the highest security, but I bet you won't loose it if you mow it into your lawn like these people did (just make sure it's encrypted):
http://blogs-images.forbes.com/davidthier/files/2012/09/img_9933_0001.jpg