What to learn as an undergraduate to better myself?

So after looking at a bunch of job postings for interns on my university website, I was overwhelmed with what I didn't know. I'm a junior in college (although I changed majors this year, trying to graduate within 4 years), and hoping to pursue a masters degree. However, I really want experience with languages and systems that will help me in the real world as well.

What are the top priority programming languages or systems that I should start learning? I'm hoping to learn a lot over this summer, especially if I fail to find an internship. I currently only know C/C++.

If there is a better place to ask questions like this, please let me know..