Cardboard. An awesome material, great for boxes and backing paper pads.
Also useful for more advanced projects.
Here are two how-to’s from the web: a simple frisbee, and a badass surfboard.
or, Cross-sectional Notched Rib Assembly for Aquatic Sliding Boards
I’m building hollow, cardboard core surfboards. Cardboard, seriously. These are ridable prototypes for a new approach to building boards out of interlocking cross-sectional ribs cut from sheet materials.