We try to incorporate our kids' interests into their schoolwork, so lately I"ve been keeping track of their favorite subjects!
My son loves dinosaurs, astronomy (stars, solar system), and superheroes. Now that I write it all out, I guess he's the stereotypical boy in that sense. He is really into adventure of all kinds.
My daughter has picked up a lot of her brother's interests, like superheroes (but usually female ones). But she has a huuge love of all animals, just like her mom. Her brother loves animals too, but she takes it to a whole new level.