Start doing some part-time businesses now. Get with classmates, especially classmates outside your major, put together something quick and dirty and see if you can make a few bucks. You'll learn what you need to complement your skills & personality and get some experience balancing different parts of your life, going through the whole business start-deliver-grow-falter-fail cycle. Invaluable experience and killer resume/interview material.

For social groups, find groups _outside_ of college. You'll be able to connect with people at all stages of life, and you won't be an outsider the day you graduate. Find something with a worldwide presence, whether it's the group or just the activity itself. I did medieval re-enactment, I connected with not only other medievalists worldwide, but all historical enthusiasts. Travel? Free room and board and often transport from the more established co-enthusiasts. Instant networking, too--jobs, advice, backing, prospects, you name it.