The CPH metro goes all the way out to the airport and is a wonderfully easy way into and out of the city. It's definitely worth the ride, if only to have a walk around. CPH in summer is a truly beautiful and enjoyable place! There's a harbor tour by boat that boards a few blocks from the Kongens Nytorv (as I recall?) metro stop and takes maybe 45 minutes. Good fun if it's a sunny day. If you walk over to see the Little Mermaid, the old military fortification 'Kastellet' is nearby and is (partly) open to the public. It's a terrific place for a picnic.