Ranma 1/2: Battle Renaissance (1996)

It's a normal day at the Tendo Dojo when Kunou invites the Ranma gang to Kikaiga island. The Kunou family has bought the island and is building anamusement park on it. When the group arrives, they find that Kikaiga island is a demon's lair, and that a legendary treasure called Shingouki is hidden within eight stones throughout the island. And so the characters each go out to search for the stones and claim the Shingouki before anyone else.