The National Aquarium Denmark, Den Blå Planet ("The Blue Planet"), is a sea of fun for visitors of all ages. As northern Europe's largest aquarium, it offers a journey under the sea with thousands of fish and sea creatures in 48 aquariums. And for some reason, there is also a rainforest! The aquarium also features a large cafe and a really fun playground outside. This is a great indoor option if you are visiting with kids! Note that it does get very crowded during school holidays.