Ask HN: What technologies should I be learning with my 20% time?

So I'm a student, and this summer at my internship I'm being given some form of "20% time". Ideally, I'd like to use this time to make myself more work-ready for when I go on a 16-month internship next year. What technologies would you recommend learning for a still-learning student interested in embedded systems and back-end development?

Thanks!