I doodle cars, tags, cartoon faces or just random squiggles. I did a whole doodle art piece at college on an A2 sized piece of paper and it's amazing what you can actually create by just zoning out. I had crocodiles sunbathing at the beach - don't ask, lol
I doodle random shapes and small figures mostly. I tend to do this at long meetings when a lot of other people are talking - helps me stay focussed, particularly if i'm feeling tired or somewhat bored :-).
I doodle knights, people, sometimes 19th century objects. I do this usually in the back of a boring class at school. Doodling helps you concentrate, for all of you that didn't know that. It takes the mundane out of well, mundane school.
I'm working on a light novel and a manga douji right now, so when I get bored I just doodle concept artwork for the characters or buildings, or technology. I don't have to get too heavy on detail because they're just concepts, so It really helps relieve my boredom.