1. Learn how to write: It is tempting as a science major to focus only on technical skills, and learn only how to do the math. While that is the path to your best grades, it's not exceptionally useful in most walks of life (being a theorist is an exception). However, you'll find that your success will often be based on how well you write: your statement of purpose for grad school, the papers you publish, documentation, presentations, etc. Writing clearly and quickly is a very useful skill, and a skill that is often neglected by technical majors (while I worked at CERN, a surprisingly large portion of my time was spent writing documentation).
2. Learn how to code: you don't have to become an algorithms expert, but you should learn the basic skills of coding, such as debugging. There is a good chance that your technical degree will put you in a job where coding knowledge will be useful.
3. Have fun! Getting drunk at a frat party might seem like a waste of time, but you might meet your future wife/husband there, or at least your future girlfriend/boyfriend. You also might meet future business/academic connections. Honestly, people don't change much in this regard, I've gotten to know several profs fairly well, and most of them are the result of drinking with them.
Of course, if getting drunk isn't your cup of tea, you can still get many of these same things out of more tame forms of social activity. Many of the more fun moments of my undergrad were also completely sober moments.