Ask HN: What Should I Focus on Learning Over the Summer?

I'm currently finishing up my first year of college for software engineering and I plan on taking the initiative to learn on my own over the course of the summer.

My question is: What should I focus on learning over the summer?

The one thing I know I do not want to learn is Java, as it is the one language I am already familiar with, and will continue learning when I return to school in the fall. I'm interested in developing for the web, but my goal is to just become a more knowledgeable, and better developer/programming by the end of the summer. What do you guys suggest I do?