I do this type of exercise often, usually when I need a break!
The exercise I like most is:
1. put down almost random paint, in three or four colours max. Ensuring there is some overlap. I use acrylic or watercolour mostly.
2. I find lines running across the page and through shapes. I trace these in 2 contrasting colours - often black and white - in different widths.
3. then, I get my full pallette and look for shapes that could become objects - heads, legs, buckets, birds, buildings, ... and use various line options to either highlight the paint already there or recede it.
4. last step involves looking at the whole image to create from connections and contrast on the page.
Hope that helps. Here is one I prepared earlier ... my goal here was to create some type of 360 degree views, and emphasize white.