After all the lectures and homework, many of us would rather not look at another book or hear another word above 4 syllables. However, I think it is a big mistake for a college student to stop learning outside the classroom. For one thing, doing extra research into the field you are majoring in can give you a better idea of what your career would be like. Whatever you learn can help you develop a passion for your major, or make you realize that you desperately need to switch majors. In addition, you can explore which subdisciplines you want to specialize in and pick your electives accordingly. If you are particularly ambitious, you can learn about the cutting edge of your field which could foster some highly profitable ideas down the road. Or you can learn about subjects in other fields and find creative ways to incorporate it into your field, thus making scientific/technological advances. Besides helping your career, independent learning is a valuable, lifelong skill. It's like reading. If you do it often enough, you'll be able to do it with greater speed, perseverance, comprehension, and retention. College is the perfect time to develop this skill because it offers many resources that are "complementary" to your tuition.
Your typical campus library will house way more books than your average public library, plus it'll let you check out books for longer periods of time. It's the best place to find non-mainstream books on science, psychology, economics, etc. If you haven't already, you should develop a habit of reading nonfictional, academic literature. For a STEM major, it's important to be able to comprehend technical jargon and complex mechanisms. For a citizen in a democracy, it's absolutely essential to be "well-informed". When you are just starting out, there are lots of introductory books that can get your feet wet. However, even the introductory books can be biased if there is controversy in that field. For example, I'm currently reading Basic Economics by Thomas Sowell. As informative as he is, most of the book is an argument for free-market capitalism. I'm ideologically on the same side as him, but I know that I'm going to have to expose myself to the opposition's arguments before I can fully justify my position on this subject. As with any other source of information, be critical.
Because you are affiliated with a university, you have access to thousands of dollars worth of papers from databases such as ScienceDirect. Like the library books, reading the papers can help you understand the technical jargon and complex mechanisms. The papers can be very difficult to read. So, I recommend printing each paper at the campus library and annotating it. You should be able to understand what the researchers' area of interest, goals, methodology, and findings. Plus, reading the papers should help you improve your graph reading skills. Because the high-cost subscription journals tend to be more rigorous than their open access counterparts, you should also use these paper to develop a standard for scientific (or any other) studies. You might continue reading open access journal papers after college, so you will need to judge the credibility of the paper.
You can get these for free at your major's department. I have also been able to get a free subscription to two magazines by the American Society of Civil Engineering just because my university has a branch. I didn't even have to be a member of the branch to be eligible. These magazines are a great resource for learning the new developments in the field. If you are interested in doing R&D work, the magazine published by the university can give you an idea of the current hot topics. I even found the job advertisements useful because they hint at what skills are in demand and what locations are hiring.
I found a license for Mathematica and Matlab on the dashboard in my student account. In addition, the Engineer Computing Center offers other software such as Cadence. These are likely the tools you'll be using in industry, so you'll benefit from playing around with them even if none of your current classes require it. You can also come up with some cool projects for using these software programs. For instance, Mathematica is a great toy for Big Data enthusiasts. Matlab can be used for controlling Arduino boards or robots.
The campus events are great places to encounter new ideas. There are guest lectures where anyone can drop in and hear an expert talk about their area of interest. With the more technical ones, it's a good idea to read the abstract and do some research beforehand. An added bonus is that there's often free food. You can also go to seminars that will teach you certain skills. For instance, my university's Engineer Computing Center offers workshops for programming and using Linux OS. Besides that, there are also discussion panels and debates.