To do all the sightseeing in Denmark grand Copenhagen area need more than 3 days. As you know, the cost of living in Denmark is very high so in order to save cost, we do recommend you use Copenhagen Card. This card can offer you free admission to almost every important sightseeing spots and museum in Copenhagen area. Besides, you can use the bus, train, metro, all transportation free. The card is not cheap but I did a calculation and found that this card can help me save up all the transportation cost, i.e. the admission fees that I have to pay for my visit are good enough to cover the cost of Copenhagen card.
Hence, we bought a 3-day card about 90 Euro and as you expect, we would squeeze all those tourist spots that need to pay admission fee into these 3 days. Therefore, on the first day we were in Copenhagen, we went to all sightseeing points that are admission free.
After we went to Frederiks Kirke, we walked to Kastellet, an old barrack for Denmark but it is still in operation today. They open a part of it for visitors. The barrack is in star sharp and guards are standing at the entrance. It is very quiet inside but nice and beautiful buildings are found.