I'm reading Craft-a-Doodle: 75 Creative Exercises from 18 Artists. Some of the exercises get tried out in my sketchbook. The first two are self-explanatory:
This next one really made me stretch. First you use a small inkpad to make blocks of colour all over the page. Then you add onto them with a pen. After the first twelve, when you can't think of any more, you're encouraged to do twelve more, because that's when the best ideas come!