Cheat Codes:
The following cheats are entered at the Cheats screen on the options menu: 
Infinite Juice Bar: BAKABAKA
All Keys: SKELETON
Invincibility: KHUFU
Perfect Manuals: QUEZDONTSLEEP
Perfect Grinds: BIGUPYASELF
Prefect Handplants: JUSTIN BAILEY (Note the space between N and B)
Full Stats, Level Select and all Park editor objects: Up, Up, Down, Down, Left, Right, Left, Right, BABA 

All Characters: Down, Right, Right, Down, Left, Down, Left, Down, Right, Right, Right
Low Gravity Wallrides: Up, Down, Up, Down, Left, Right, Left, Right, ABABS
Super Spin: Left, Left, Left, Left, Right, Right, Right, Right, Left, Right, Left, Right, Up
Juice Faster: Left, Left, Right, Right, Left, Right, Down, Up, Up, Down, AI