Cardboard seems to be the junk art material of choice for many people. It's easy to acquire, has many possibilities (melt it with water, peel back the layers, paint on it, layer it, can be strong enough for support of heavier objects, etc) and its color basically screams to be covered in something. It's so cheap and easy that it's a relatively guilt-free material to experiment with.
When I saw this Cardboard Surfboard, I was intrigued. The creator of this design posted directions and blue prints on this site so you can make your own and help improve it.
(image credit to linked article)
Not sure I would trust a cardboard surfboard but it is pretty interesting!