Dogs, cats, horses, and eyes (hard to master eyes and I'm not getting close), cartoonish things, and geometric shapes. I doodled the 3 dimensional cube once and got stuck on the phone, so I just kept adding to it and by the time I was done I had drawn the floorplan of the Winchester mystery house.
Geometric shapes too. Usually squares and triangles of some kind, then I color them in too. I also like to do a bunch of squiggles then color in the shapes too.
That's what I doodle too. I also color in the spaces in the letters on the paper I'm using. I color them in on the calendar too. An addiction like popping bubble wrap?