The exhibits are really thoughtfully designed, recreating different natural ocean environments. Each area makes you feel like you're actually in the animals' natural habitats.
Korakuen has these wide, open lawns, so if you stand on a higher spot, you can see the whole garden at a glance. Plus, Okayama Castle is nearby, so you can visit both together.
If you want to check out the Harry Potter area, you should head there as soon as you enter the park and get in line. There are crowd controls, and you'll totally regret it if you miss out.