Others have suggested great things to do outside of class, but I have some related to CS.

Take a compiler course (or several). This really should be required, but unfortunately it wasn't at my school until after I graduated. Also, lots of algorithms and other "real" CS courses. Now is the time to learn that stuff.

Avoid the "trendy" courses (Stanford Facebook application class, I'm looking at you) and go for the hardcore CS stuff. The rest you can easily learn on your own, but I found having a professor and structured course was invaluable for the CS topics.

Of course everyone is different, but that's my opinion.