Explore mid-level classes, especially in your major. If you have already picked a major begin taking 300 level classes that are within the criteria of that major. If you have not picked a major, chose 300 level classes in different fields in order to help you discover where your interest lie.
Begin writing your resume and update as you go. It is important to have a resume and to keep it up to date at all times in case internship or job opportunities arise.
Continue to cultivate professor relationships. Professor relationships are important because as a student you will have a better learning experience. Also your professor will know you well enough to write a letter of recommendation if you need one in the future.