Boring as it sounds, I'd probably just sleep more. I don't get enough sleep...
Other than that, I'd get outside more. With a while two extra days, I could go camping every week
And hiking, and swimming and stuff.
I suppose I'd have to study a little more, too, when the quarter starts, or at least NOT wait until the last minute to do homework & such.
Or...no, wait. Better idea: I'd finally learn to play guitar. Yeah. That's the one I'm going with.