Play as Dominique 
At the main menu, highlight the "Options" selection and press 
Left or Right and choose the "Codes" selection. Then, enter 
"DOMINIQUE" at the codes screen to unlock Dominique, the 
best skater.

Play as TP 
At the main menu, highlight the "Options" selection and press 
Left or Right and choose the "Codes" selection. Then, enter "TP" 
at the codes screen to unlock TP. TP is a mummy with 95% in all 
attributes, with a bike called Pharoah.

Get the skateboarder lugnut 
To get the skateboarder lugnut put lugnut 
in at the codes screen.
From Kishkj

Enter the following as passwords

Code -- Result

geep -- Alien
nyub -- Alien
bink -- Alien
redline -- Red car
whiteline -- White car
blueline -- Blue car

From Chalmers Thopmson