ASK HN: One whole summer to learn/work on something new: what should I do?

So, I just recently finished my third year of college and now have three months of free time on my hands. I'm working toward a CS degree, however I'm a little behind because I spent my first two college years taking some Biology courses (I originally wanted to go to medical school but... nope haha). Anyways, I didn't have enough CS classes completed yet in order to find myself an internship (I'm located in MN), and now I'm looking for something useful to learn/something cool to build with my free time. As of right now, I've only learned Java and Javascript through my school, but I've taught myself Mysql and PHP on the side.

Any tips/ideas on what I could learn this summer? I'm really interested in web applications and maybe even some iOS apps, but to be honest I like learning anything and everything I can. Thanks guys!!