From the Super Mario Wiki
Koopabiza is an island appearing only in the Club Nintendo comic "Super Mario: Erholung? Nein, danke!" In this story, Mario, Princess Peach, Yoshi and Luigi take a vacation to Koopabiza. It seems to be a typical tourist location, however, Mario and his friends are deeply disappointed about their holiday. The only known place on Koopabiza is the Gumba Royal Hotel. Bowser and Morton Koopa Jr. work on the island.