Drøbak Aquarium is a small harbour-side aquarium devoted to the life of the Oslofjord. It is not a grand national aquarium, and it is better for that: the tanks show the fish, crabs, shellfish, and other creatures that live in the water just outside the door.
It is especially useful with children or on a wet day, but curious adults will get something from it too. Pair it with a harbour walk and Oscarsborg rather than treating it as the main reason to come.