Enter the following codes at the 'Main' Menu

Unlock Box Car Vehicle:
B, B, Y, X.

Remove the Maximum Top Speed on the Car:
B, B, Y, Y.

Unlock Side Characters have Chicken Heads:
A, B, B, X. Unlock Japanese Crash:
B, X, B, X.

Unlock Baby Crash Costume:
Collect ALL the Die-O-Ramas.

Unlock Nina Cortex:
Give her the keys at Midway.

Unlock Pasadena Opossum:
Give him one Power Crystal at Midway.

Unlock Third Tier Kart Weapons:
Spend 1 Power Crystal at Midway.

Unlock Von Clutch:
Give him 1 Power Crystal at Midway.

Unlock Crunch Bandicoot:
At Midway give him 2,500 coins.

Unlock N. Gin:
Give him Green Plutonium at Midway.

Unlock Crash Skeleton Costume:
Talk to the Astro Land worker near the stunt arena and buy the costume for 4,000 Coins. Unlock Evil Crash Costume:
Find Cortex on Mystery Island and behind him there will be an owl-shaped lamp. Use the body-slam switch to raise the lamp and then enter the revealed cave and go to the right. Talk to the drone at the top of the small island to get the Evil Crash costume for 2,000 Coins.