I spent many of my student holidays hacking on stuff. That was really great and I learned many technologies/languages that we never touched at university. But

I spent many of my student holidays hacking on stuff. That was really great and I learned many technologies/languages that we never touched at university. But I also did two internships, one at a big, one at a small company. They were incredibly valuable : I learned about real-world (unit) testing, code reviews (by people more competent than me), working with a big infrastructure, having deadlines, etc... Those are all skills that are really useful for any programmer and they're helping me now that I'm working on a project that I hope will become a startup.