1. Campus Events
3. Professors (yes!)
4. Career Centers
5. Walking around campus
I would add to Ashley's list, "6. Leadership Opportunities" such as student government, clubs, and organizations. These allow students to become familar with volunteering, participating in committees, learning Robert's Rules of Order, etc.
I encourage all students to take advantage of the opportunities within the college or university. Access to vast amounts of resources, libraries, and scholarly professors can be one of the most rewarding experiences.