The aquarium is nested just in between Mt. Takasaki and Beppu Bay.
If you are visiting Oita with your kids, you have to stop by Oita Marine Palace Aquarium. The place hosts an area where children can pet and play with tortoise and participate in a dolphin show. The performances are epic and the main water tank "Daikaiyu tank" includes over 90 types, and 1500 fishes. You can truly spend hours staring at this thriving undersea life and not get tired of it, if you stand on the glass floor you could almost feel like you are down the ocean.
This is definitely a magical experience you would like to share with your family and you will spend a lovely half day exploring the alleys of the aquarium. As a bonus, the place is mostly covered which makes it the ideal attraction to visit on a rainy day.
The aquarium is actually a great visit combination with the Monkey park facing it. You can even get a combined access ticket at the Tourist Information Centre when you arrive at Beppu station. The ticket will offer you a bus ride and entrance to both attractions for a lower fee.