It depends. Being autistic and whatnot, I get focused on a specific interest at a time, and that usually becomes the subject of my art for that period of time. In the past I've drawn gears and mechanical parts, robots, aliens, zombies and even the old nursery rhyme character, humpty dumpty.
Right now, I have a deep interest in the comic book character The Joker, and usually doodle him in my school notebook during classes. Who knows what I'll get into next.