You could sign up for math mooc classes at Coursera or edX, or elsewhere if you prefer to learn alongside others. If you are okay with learning on your own, you could pick up some books on math. I would suggest topics like linear algebra (believe me, this is useful for multivariable calculus), or statistics/probability to expand your horizons a bit, or you might study differential equations which will give a broader perspective on differentiating functions. You could do linear algebra and/or DE's in MIT OCW.
One thing I would say, is that there is no question calculus knowledge will go stale with non-use. However, most people will find that they remember far more than they think they do once they get into the class. Therefore, I don't think you would be harmed at all by broadening your horizons and looking into some other subjects in math before college.