When I have some leftover paint, I like to add some colour and pattern to my sketchbook.
The last 2 pics above are a monoprinting technique featured on Design Matters TV as 1 of their free tasters. I was familiar with the technique but had forgotten about it. They give a very good demo and its a simple, effective method - great on fabric too - see below.
I had a piece of X ray film spare so decided to cut a quick stencil.