Inside the PlayStation 2 version of the game there is a hidden menu that can be accessed by owning 2 of 3 cross play compatible games. Notably, a lot of the concept art showcases ideas mentioned within the game that are never actually shown.
An early mech design for PlanktonA SpongeBob style level section with lots of things that don't make sense, like cows, pigs, a giant squid, Jellyfish that don't resemble what is seen in SpongeBob, and a key object.Likely a level based on RetrovilleThe exterior of a cut Casper High levelVlad's castle built on Amity High. In game, Casper High can be seen WITHOUT Vlad's Castle on top of it in the background of Haunted Amity Park, which contradicts what Vlad says in the opening cutscene for Ghost Zone PrisonFish NPCs for the first Bikini Bottom level. Only the one on the far left was used, and the opening cutscene uses different fishThe SpongeBob clam with a red color scheme, not matching what is seen in the show. Scallops in the season 1 episodes Jellyfish Jam and Valentines Day are red, but the inside of their mouth is purpleThe Jellyfish harvester with a fish driving it. The generic goblin style enemy is in its place in the gameCalamitous with an animal-styled mech, likely intended for a solo boss fight when Retroville had its own level setFull view of the Chum BucketTwo types of fairies (likely enemies) and Crocker without a mech suitMultiple Fairy NPC concepts. None of them resemble the one used in the gameTwo unused ghost conceptsA bulky Amity Park NPC that wasn't usedA meat enemy based on the Danny Phantom episode "Mystery Meat"A quadruped animal ghostTimmy with a different Wanda pack and Cosmo as a radarTimmy with Wanda as a headband + eye view lens and a different Cosmo gun. The gun itself matches what Timmy is seen with on the box art but it was colored red with Cosmo's face absent
Alongside the PlayStation 2 and GameCube releases, an Xbox release was planned but for unknown reasons cancelled. This is evident on the IGN page, which still lists it to this day.