If you wish to visit Copenhagen, you should inquire whether you will need a visa or not. If you do, make sure to file an application in due time.
If you are an EU citizen, you must either present a passport or an ID card valid for the duration of your stay in order to enter Denmark for tourist visits of up to three months.
Citizens of other countries must have a valid passport. Some non-EU citizens are required to produce a visa. Please contact your travel agent or the nearest Danish diplomatic post for further information.
Find more information at the website of the Ministry of Foreign Affairs of Denmark.