Wave Racer 64  

Change Colors: While selecting your jetski, highlight any character and
hold up on the analog stick. Continue to hold until your character and
jetski change color. 
               

Reverse Tracks: To race all the tracks in reverse, beat the game on the
Expert setting. 
        
Turbo Charge: For a faster jetski, hold the gas button immediately before
the annoucer says "Go!" The closer you are to the announcement, the faster
your ski will go. 

Ride the Dolphin: Go to the Dolphin Park Stunt Mode.  Do every single 
stunt.  I mean do a barrel role each direction and every other stunt.  
You also have to go through every single ring.  Now choose championship 
mode and then Normal, then Warm-Up.  Now hold down on the control pad ( 
not the analog stick ) and choose a jet ski.  Continue holding down until 
you reach the level.

See the Penguin: To see the penguin in Wave Race 64 you have to get first 
in every single race in Cool Wave Mode.  you get Cool Wave mode after you 
beat expert mode.

Giant Dolphin: Play and quit warm-up mode 20 consecutive times.  The 
dolphin will now be a giant one.

Baby Dolphins: in Warm-Up mode follow the dolphin as perfect as you can 
for three laps of dolphin park.  Two baby dolphins will then start 
swimming will the other dolphin.

Ride backwards: To ride backwards, rotate the joystick clockwse and quickly
press and hold Down.