I definitely recommend spending a day at the Osaka Aquarium (Kaiyūkan) to anyone traveling in Osaka for a couple of days. It is a great place for adults and children alike and a good option for a rainy day. It is one of the largest aquariums open to the public in the world.
Focused on the 'Ring of Fire' theme, all tanks concentrate on animals and fish living in the Pacific Ocean. Upon entering the aquarium, visitors walk through a tunnel-shaped tank (about 11 meters long) filled with fish and small rays. This first tank is called the Aqua Gate. From there 14 other tanks follow, each designed after an area along the Pacific Ocean. The otters from the Aleutian Islands are extremely playful and there is always a large crowd of kids watching them. The seals, California sea lions and Pacific white-sided dolphins are also a must-see. The Great Barrier Reef tank is packed with colorful fish and imitates the coral reef of Australia beautifully. The most impressive tank is about 9 meters deep and is the home of a whale shark and some manta rays. If you visit the aquarium around Christmas, you might be able to see Santa in this tank swimming among the big fish. Finally, another must-see is the Japanese giant spider crabs which can get as big as 3 meters wide!
The aquarium itself is well built and seats are often available to sit and enjoy the view. It can get very crowded on weekends, so I recommend going during the week. It might be difficult to read all signs depending on the number of people, but it is possible to rent audio guides at the entrance for a relatively low cost. I personally think simply walking and watching the animals and fish is enough. Kids with Nintendo DS can also access the video guide information with their console. You can easily spend 1 or 2 hours at the aquarium. Also, the museum has a shop selling a variety of souvenirs and toys for children.
As for restaurants and coffee shops, the aquarium is located near the Harbor and the Tempozan Harbor Village. It is relatively easy to find something to eat close by for all budgets.
Tickets are about 2,000 yen for adults and 400 yen to 900 yen for children depending on their age.
To get there: From Osakako Station it is about 5 minutes on foot.