Go to the main menu, select the "Extras" -> "Enter Codes" and enter one of the following codes. Then choose "Activate Bonus Items" to activate the cheats:
30,000 Z: ROCFISH
Bonus vehicle in Diesel Dreaming level: HOTROD
Bonus ship in Rocket Rodeo level: SPACE
Alternate Plankton hovercraft: HOVER
Play as Punk SpongeBob in Diesel Dreaming level: SPONGE
Play as Tuxedo Patrick in the Starfish To The Rescue level: PATRICK
Play as SpongeBob Plankton in Super-Size Patty level: PANTS