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, B, A, B, A 

All Characters: Down, Right, Right, Down, Left, Down, Left, Down, Right, Right, Right 

Low Gravity Wallride: Up, Down, Up, Down, Left, Right, Left, Right, A, B, A, B, S 

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