Hi,
There are many outdoor festivals in Denmark in the summer.
Most notably is Roskilde Festival, a week long festival near Copenhagen, which is the biggest music festival in the Nordic countries. Most people there are Danish, but it also gathers an international crowd.
Copenhagen also several, such as Distortion, which is a different vibe. While it doesn’t fit my music taste, I know some people who are excited about Copenhell. If you want something more “down to earth” there’s Copenhagen Jazz Festival.
There’s also Smukfest in Eastern Jutland and Tinderbox near Odense. They well known too, but I haven’t been to these. And there’s countless of smaller festivals all around the country.
Students mostly eat food they prepare from ingredients bought at a supermarket. It’s all they can afford - unless they have a student job and make the occasional splurge. For street food, you’ll find many options in Copenhagen, including (but not limited to) Torvehallerne, Broens Gadekøkken and Reffen.