Yeah, it sounds a little Calvin & Hobbes, but riding around with corrugated cardboard on your head can actually be safer than the plastic and Styrofoam concoctions you get at the bike store. The Kranium cardboard bike helmet absorbs four times more impact energy than equivalent polystyrene. One helmet was smashed five times in a row and still had enough muscle to pass a standard safety test. And yeah, it's waterproof.
Kranium helmets can easily be cut to a custom fit by scanning your head measurements, but the helmet's designer, Anirudha Surabhi, also envisions a world in which you can buy cardboard head-armor out of a vending machine.