You should visit the Popkomm' and the 'Ringfest' in August in the city.
Every year there is a big music fair and beside this fair is the 'Ringfest'. There are many stages where you can see and listen to live music of famous singers and bands. It's great fun and the music is absolutly great. But it's also very full and you have to walk through the huge crowd. But hey it's cool.