Welcome back to Rocky!
Now that you're a sophomore, you may be feeling a bit like the world is your oyster. You already know how life works at Princeton, you may have some sense of the direction that you'd like your courses to take this year, and if you're an AB student, you're beginning to think about your concentration (you BSE folks already have that down, too.)
So: now what?
Get to know some professors. This isn’t to say you need to get these professors to write you letters of rec, but you should get to know the people who work in your department and who can recommend you classes or help you with research down the road. They are generally fabulous human beings with a wide range of interests, and they like to talk with students like you! It's not at all unusual to invite one of them to lunch. Become involved in departmental activities, such as guest speakers, receptions, symposia, committee work open to students. Start to imagine what it would be like to be part of this community.
Build your leadership skills. Take on more responsibility, but make sure to balance that responsibility. The art of balance is probably one of the most important skills you'll take from college.
Visit some other offices around campus to find out how you can take advantage of the resources available to you. Now is the time to start exploring internships (Princeternships and IIP are fabulous), study abroad, fellowships, and other opportunities to enrich your undergraduate career. Not everything you will learn at Princeton will show up on a transcript.
Mentor someone. Nothing can reinvigorate your love for your college like passing your knowledge along to someone new. Find out how you can help other students. This experience will very likely help you focus on your own values and passions.