Hidden Cups
Zenith Cup - Go to password on the settings screen and type in the code DZPHHH.

All Cars and Levels
To get all cars and levels enter the password WLLYDD.

 Hidden Cups
 All Cars & Levels
 S.C.A.R Cheats
 Secret Cars
S.C.A.R Cheats
All Cars: WLLVDD
Crystal Cup: LGSSSX
Dimond Cup: CRKKYY
Master Mode: PXPRTS
Credit Goes To: C1Outrider IGN 64

Secret Cars
Cheetah Car - Enter the password NRNNRR
Cobra Car - Enter the password TRTTLL
Scorpion Car - Enter the password SDSSSRT
Panther Car - enter the password YMSTTR
Credit Goes To: Brad Otherman via N64cc & D-Man via IGN 64