I have a similar request. I'm a rising junior majoring in Chemical Engineering -- I'll be working at an internship this summer and will be learning Java, but will probably have a decent amount of time left over.
Most skills that I have of note are related to my major. Much of my spare time has been devoted to reading lots of books, mostly centered around psychology and business.
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.