The Concept Art for the game (unlocked by paying 40,000 shiny objects to the theater) shows art for a proposed Robo-Squidward boss. While it was dummied out of the console games, the Game Boy Advance version did include it as a boss.
Also, there's an unused level found in the Gamecube version, said unused level is what supposed to be Patrick's Dream. Said level also contain several early version of in-game objects. See it here
Glove World would have been it's own world, judging by an unused title seen in an early version of the pause menu
Development Gag: Although the full Patrick's Dream level didn't make it into the full game, he still gets one where he just sort of gives you the Spatula, with him standing in a blank black room with a spotlight being the only immediate illumination. This is more likely a reference to the episode the world is based on, but it can double as Self-Deprecation.