Christiansborg Palace is the building of the Danish Parliament. It is a very nice historical building. The kitchen and the stables are also open to the public so anyone can go see them. You can go up the tower for some amazing views of the city. Going up the tower if free of charge but you have to go through security, which might take a while if there are a lot of people, because only around 6 people can go at a time. The building also has a gift shop and a restaurant. Very nice place to visit when you're in Copenhagen.