Here are a few pages from my Sketchbook Project. My theme is: Your Name Here.
What fascinates me about this project is that this is the first time I've created a sketchbook with the intent that it would be viewed by many people and the first time the entire sketchbook had one theme. I haven't finished the book so I don't know what else is going in there.
Usually, my sketchbook is to play with design ideas, sketch whatever is in front of me (a great exercise when the brain isn't being as creative as I'd like), and to play with color. Many times, I've found that an idea in my head turns to dreck when it's on paper. The sketchbook saves me from wasting time on a design that just isn't going to work.