For patrick skeleton go to the extras menu in the main menu & select cheat codes (THE ICON IS KEY) and then follow this cheat 'BONES' for Spongebob brain costume follow this cheat 'BRAIN' (THIS COSTUME IS ONLY FOR SPONGEBOB AND PATRICK NO PLANKTON AND THIS ONLY FOR HYPNOTIC HIGHWAY)
To be able to enter the following passwords go to 'Extras' and then 'Enter Codes' and type them in (without the quotes). To activate the the cheats go to the 'Activate Bonus Items' selection under the 'Extras' menu.
Unlock Level Select: Enter 'GUDGEON'.
Unlock ALL Bonus Games: Enter 'PORKPIE'.
Unlock ALL Sleepy Seeds: Enter 'SCOOTLES'.
Unlock Sleepy Seed Detector: Enter 'TISSUE'.
Unlock 30,000 Z: Enter 'ROCFISH'.
Infinite Fuel in Flying Levels: Enter 'GASSY'.
Infinite Health in Platform Levels: Enter 'VIGOR'.
Unlock Bonus Vehicle in Diesel Dreaming Level: Enter 'HOTROD'.
Unlock Bonus Ship in Rocket Rodeo Level Enter 'SPACE'.
Unlock Alternate Plankton Hovercraft: Enter 'HOVER'.
Unlock Alternate Rocket in Patrick in Hypnotic Highway Level: Enter 'ROCKET'.
Unlock Alternate Laser Color in Revenge of the Giant Plankton Level: Enter 'LASER'.
Unlock New Effect for Patrick's Star Spin in Starfish Man to the Rescue Level: Enter 'SPIN'.
Play as Punk SpongeBob in Diesel Dreaming level Enter 'SPONGE'.
Play as Astronaut Plankton in Revenge of the Giant Plankton Level: Enter 'ROBOT'.
Play as Pirate Patrick in Rooftop Rumble Level: Enter 'PIRATE'.
Play as Tuxedo Patrick in Starfish Man to the Rescue level Enter 'PATRICK'.
Play as SpongeBob Plankton in Super-Size Patty Level: Enter 'PANTS'.
Unlock Safari Patrick Costume. Enter 'SAFARI'.
Unlock Skeleton Patrick Costume: Enter 'BONES'.
Unlock Inventor Plankton Costume: Enter 'INVENT'.
Unlock Aviator SpongeBob Costume: Enter 'PILOT'.
Unlock Exposed-Brain SpongeBob Costume: Enter 'BRAIN'.
Unlock Skeleton SpongeBob Costume: Enter 'KRABBY'.
Unlock Patrick's Purple Rocket: Enter "BUNRUN" as a code to unlock Patrick's purple rocket.
Unlock SpongeBob's Flame Effect Colour: Enter 'FLAMES'.
Unlock SpongeBob's Hypnotic Car Skin: Enter 'HYPCAR'.
Unlock SpongeBob's Squeaky Duck Gun: Enter 'DUCKGUN'.