Art History
Animation isn't just about motion. It's about capturing the impression of life in a stylized fashion, sometimes simply, sometimes with complexity that almost perfectly mimics the real thing. There are as many ways to do this as there are great artists throughout history. Studying how historical artists captured life in their drawings, the different methods they used to evoke emotion, tone, body language, depth of field, and movement in still images alone will help you work out a good way to capture life in your animations, and develop your own unique methods.
Graphic Design
This one is probably the most obvious, yet you'd be surprised at how often people overlook this. Graphic design teaches you about certain things that simple animation and art classes don't; it's a different approach to art, one focused a great deal around style, color theory and balance with the goal of producing commercial designs. You'll learn to look at your animations and character designs from a different perspective, and how to add a certain level of harmony to your art that gives it a more polished, finished feel.
Human Anatomy & Biology
Not just a drawing course on human anatomy, either. A real, honest-to-goodness human anatomy and biology course, preferably one targeted towards med students. Get in there, right down in the meat of it. Learn the human body from the inside out. Learn what makes it tick, what makes it push and pull, until you know it so well that you could draw it in your sleep. Understanding how the body works in all its complexity will help you capture realism when you're drawing it in its cartoon simplicity. It might even spark an interest in medical animation.
Animal Anatomy & Biology
Animals don't move the way people do. They aren't put together the way people are. You can watch Animal Planet all day and never quite understand the complex interactions of musculature that creates that particular motion we identify as feline, serpentine, or any of the distinctive movements we associate with certain animals. An elective class in animal biology can help you understand their skeletal shapes, how to draw them, how they work from the inside out, the same as with human anatomy and biology.
Mechanical Drafting
Drawing machines is at once insanely difficult and ridiculously simple. Machines are generally uniform curves and straight lines, easy to draw, but interlocked into a complex matrix of moving parts that require a great deal of perspective work to draw properly. You'll learn a lot about drawing machinery and mechanical objects in a mechanical drafting course, more than you will in the week or so you might spend on it in a general art course. You'll be able to apply that when animating mechanical objects, so your cars will actually look like cars and not balloon animals, and your giant mechas will look like fearsome robots instead of poorly-cast plastic mold toys. Want to take it a step further? Take auto shop. Don't just learn how to draw machinery; learn how it fits together with hands-on practice, and apply that knowledge to improve your drawings.
Theatre
Theatre is about storytelling. So is animation. Participating in theatre can teach you a lot about how to plan a story in all its acts and scenes - but there are added advantages, as well. Working with props and sets can teach you about laying out scenes. Working with stage lighting can teach you about painting in the proper lighting in your animations to set the tone. Watching actors and actresses learn to convey emotion with their voices, expressions, and body language so they can be understood even in the furthest balconies can help you learn how to infuse that emotion into your animated characters. You can further enhance that by branching into cinematography and film, especially video editing and special effects. Learn the process of filmmaking, the many different tricks that go into creating compelling visual imagery augmented by sound and music, and think about how these techniques can be applied in animation. Just because they're different media doesn't mean they have to play in separate sandboxes.
When you think about it, almost any course can be useful to animation - from horticulture to accounting, because the courses you study are about things that are part of everyday life. And like I said before, animation is about capturing life. So whatever interests you can be used to enhance your animation studies; it's just a matter of taking what you learn and applying it in innovative ways.
