This place was packed of great fun! Definitely a must in summer but also winter.
It was so cheap for 480 YEN we got entry to different onsen themes such as European and Asian onsen. Entry fee also had access to the pool recreational area where kids and adults can swim in the pool, under caves and ride the rides. The rides were actually fun even for adults. For 300 YEN you can go once or 1000 YEN unlimited which I do recommended.
You can spend at least 2-3 hours here. If you aren't comfortable with being naked don't worry they have a family onsen area, which my brother spent there. Heaps of food stores in 2 different levels as well and small arcade games. You see kids running around everywhere enjoying themselves.
Onsens are a must try, don't forget there are different temperatures for the onsens and different styles. They had single bathtubs as well so you could lay in there for hours then enjoy the many saunas they have as well.
Bring small change as well for shoe and lockers! More