I always keep printer paper at my desk. That way when I'm waiting for something to load or in a cutscene when I'm playing a video game I can doodle. When I'm just messing around I usually like to draw girls, they are curvy and fun when you're just idly sketching. Sometimes I draw people from my imagination, sometimes they are characters from the game I'm playing. I thought I'd post some for fun, so here is a batch:
I have piles of these and usually just end up throwing them out. Sorry about the quality, I just took pictures with my iphone. Anyway, it's amazing how much doodling you can do while you're waiting for a large file to save or sitting through a cutscene. I draw other stuff than ladies, but ladies are a lot of fun. Plus since nothing is sacred when you're sketching, there's no pressure!