The Sirena Beach area in Super Mario Sunshine, my favorite area from the game! Photo courtesy of mariowiki.com
This is one of those easter eggs that when you hear about it, it doesn't really make sense, but when you see it you go "Oh! I see it now!"
Sirena Beach is an area you gain access to towards the end of the game that is partially shaped like a Gamecube controller! The two big circles make up the upper part of the controller, while the left of these circles has the A, B, X, and Y buttons, and the right being the control stick. Below that are the two small huts that represent the D-pad and C-stick. A torch marks where the start button would be, and beach chairs are located where on the controller says "Nintendo Gamecube". The whole thing is shaped as if the bottom of the controller would be pointing away from you, and the hotel isn't a part of the controller.