The Enoshima Aquarium near Kamakura (website in Japanese, but English pdf available for download) is a great place for adults and children alike.
The price for adult admission is ¥2100 and about half that for children.
There's many wondrous ocean creatures to be seen including dolphins, seals, jellyfish, penguins and other fish. You can also see the penguin's life show as well as a dolphin show a few times daily.
The gift shop has many different souvenirs for purchase to remind you of your visit.
I recommend everyone to take the time and enjoy the aquarium, expect to spend half a day surrounded by fascinating creatures.