Ask HN: Optimal use of time to build skills for future internships?

I've built a few toys with Rails, and am pretty comfortable with Python. I've done a bit of Haskell academically (mind-bending, but quite fun), and got a taste of systems programming in C in an intro to systems class. Decent grades, a few summers of desktop support/fibre/telephony.

I'm currently working on fairly CRUD-centric Rails backend stuff for an equally inexperienced team (coworkers are also students and iOS-focused, boss is transitioning from Windows network admin work). It's decent work, but not the stuff that technical interviews are made of, and mentorship opportunities are limited.

I've got the whole internet in front of me, a few free hours a day, and no demands on my time outside of work. But there are so many options, I'm having a hard time sticking with anything.

So, HN hivemind, how can a college student best use his free time to become (more) competitive for tech company internships next year?

(If employability were not a concern, I'd be working through SICP, but there's not exactly a merit badge...)