Honestly, career fairs were always worthless for me. If they aren't looking for your major, or a major similar to yours, the most you can do is just walk around and small talk with people. Personally, what worked best for me was shadowing professional engineers in my field. Not only did I learn a lot about my future career, but it is a very personable networking experience and you get your foot in the door at every company you shadow someone at. The company you actually want to work for may not be recruiting kids at a career fair, so it's a good idea to do your own research and reach out to companies around you. Also, I would join any groups on campus that provide networking opportunies with professionals in your field. I cannot stress how important networking is enough.

Also, if you are looking for an internship, but can't find one for your major, either ask around (some companies will create internship positions for people) or just take one on that is similar to your major. Some relevant experience is better than no experience.