<p>Extracurricular activities, whether in high school or in college, are totally irrelevant for graduate admissions. The only non-classroom activity that matters for PhD admissions is undergraduate research, and you should try to become involved in research as early and as deeply as possible in your undergraduate career.</p>
<p>In terms of college math, I would advise taking at least single- and multivariable calculus and a college-level statistics course. These are the math classes required of a brain and cognitive sciences undergraduate at MIT. If you are interested in computational neuroscience, you will probably need to take additional math courses, but for molecular/cellular neuroscience, those three will be sufficient.</p>