You can see theatre, dance, music and other performances all summer, yes, for free. Best and cheapest happening every summer in parks with big stages.
Parkteatern has invited people since 1942! Last year they had 104 performances at 21 different locations.
Just be sure to come early, these events are very popular. I’m always there at least one hour before to make sure I’ll get a seat. For popular events I come even earlier. The benches aren’t the comfiest; bring something to sit on! People bring blankets, picnic baskets, their dogs, their friends, books to read while waiting for the performance to begin… It’s a wonderful place mixing all sorts of people.