If you're musically inclined, you could try busking on the streets. If you're talented you may find Copenhagen quite a lucrative city to spend the summer.
No permits are required for individuals and groups of three or less people who perform acoustic music. However you are limited to performing 7am-10pm daily at Rådhuspladsen and at Kongens Nytorv by Krinsen and Mon-Thu 4pm-8pm, Fri 3pm-8pm, Sat 10am-5pm and Sun 12 noon-5pm elsewhere in the city.
Other street performers will need permits issued by the city council and the police station.