Copenhagen is a walkable city, and most of the attractions are located within a short distance from each other. However, if you plan to visit places outside the city center, you can use public transportation such as buses, metro or trains. A single ticket costs around $4-6 per person, and you can also purchase a city pass for unlimited travel.
If you have more time, consider visiting other popular attractions such as the Ny Carlsberg Glyptotek Art Museum, the Copenhagen Zoo, or the Botanical Garden. If you have less time, you can skip some of the attractions and focus on your favorite ones. Also, consider visiting during the summer when the days are longer and the weather is more pleasant.