Sketchbooks of different sizes and proportions seem to inspire different types of drawing. I have lots of different sketchbooks going at once and there is no rhyme or reason. You can see a lot of pages from different sketchbooks on this portfolio page.
Sometimes I do free-association drawings in the evenings and early morning. No deep meaning, just drawing and coloring whatever comes to mind.
Other times I will keep a travel sketch journal on a trip, drawing things I see and making notes. When I get home, I sometimes scan the drawings and study them along with the many photos I take. These sometimes lead to a larger project such as the quilt Tokyo – Wish You Were Hair.