If you're going to Trondheim in the summer, you have to take one of the sightseeing boats out to Munkholmen. It's a tiny island right outside of the city centre, and the boat trip takes appr. 15 minutes. The history of the monastery that is situated there is quit interesting, but for those who prefer to relax, Munkholmen also has a quite nice beach, and maybe you can take a bath, if it is not too cold!
Fondest memory: Since I live in Trondheim, I have a lot of great memories from here. But I think that one of the greatest places to be (during summer) is Marinen. That is a whole lot of lawns where you can lay or sit down doing nothing! But if you are more into activities, you can bring a football, a frisbee or something like that, because there are a lot of space!
Here's a picture from last year's celebration of Norway's national holiday, May 17. It is a BIG deal for Norwegians, and the entire nation takes part in one of the parades that is so exceptional for Norway. At this picture, it's my mum that celebrates the day by entertaining the citizens, and as you can see: the sun is shining!