The Blue Planet Aquarium is one of the most impressive architectural feats in Copenhagen, designed by 3XN with an undulating aluminum exterior and revolutionary seawater cooling system. The building has five arms, all surrounded by water, and the exhibits are astounding, including the 4-million-liter Ocean Tank, where sharks, rays, eels swim behind seamless glass viewing walls. Other standouts include a coral reef, a bird and butterfly exhibit, and a waterfall tank that's home to piranhas and anacondas. Admission is 95 DKK for children ages 3-11, and 169 DKK for adults.