High school should be a time to discover your passion, and if you already have one, then dive into it. Get involved. Discover new things by joining clubs or branching out. Always work hard. That's what I would change. I would have became well rounded early on so that I had discovered my passions and followed it throughout college.
Proceeding forward with a sense of purpose and determination makes it easier to enjoy your college experience as well. You avoid the stressors of uncertainty about your major, minor, and the ultimate question: What do I want to do with my life.