Learn pickup (or an applicable equivalent). A dedicated investment of three months, 4 hours a day, developing social skills will fundamentally alter your entire experience of university, making it far more enjoyable and far easier to succeed with group projects. There is probably no better way to develop confidence than through the repetitive humiliation and sometimes positive reception from approaching and interacting with hundreds of prospective mates. The associated skills and habits of interaction developed will do far more to further your career prospects than learning any technical skills.
approaching and interacting with hundreds of prospective mates
That's a bad idea, unless you live in a city with several hundred thousand people.
This September I'll start college aiming for a computer science degree, and I want to use the summer for self-improvement. I'm very uncertain about what skills I should try to learn, though, and recommendations would help.