That's a good question. It's hard to go back to my teen years but I'd suggest:

• buy a musical instrument and learn to play it, really doesn't mater which one. Especially during college years everyone who could play anything was suddenly a superstar
• buy some book to learn some skill, again doesn't matter that much what you learn. I don't know, like mixing cocktails? Sounds fun. For example by accident I learned "reading of hands". I quickly learned that it's bullshit but boy, was it an interresting thing to perform during parties in college
• Nothing really comes to my mind regarding of profesional development. Really at your level most of the stuff you can learn is free online and you don't need anything expensive