Change Paint Job  -  In one or two player mode, when you select your car, use L and R to change the color of the car's body. 

Cops & Robbers Mini-Game  -  This isn't really a code, but it's a fun game. 

Have player 1 be the cop. 
Player 2 be the grasshopper (the supply vehicle). 
Player 3 and 4 be the monsta and the 4x4 (the getaway vehicles). 
Give the grasshopper a 10 second head start, and the getaway vehicles 4 seconds. 

The cop has to make the vehicles spin out or stop to get arrested. Race on all the tracks and keep a score tally. 

Easy Mega Roll Bonus in New York. In order to do this trick, you need to use a car that goes at about 140 MPH. Enter a Championship race in New York and as you are about to make a sharp turn, do a wheelie straight in the curved wall. You should do a Mega Roll and bounce back on course! Make sure that you are at a fast speed when performing this trick. 

Flips and Rolls  -  To do a flip off of a jump, right when you come up to a jump, press A twice. If you did it right, and your timing was right, you will do a flip, or a barrell roll. 

Jump Over Cars  -  When you are coming up behind a car very fast, pop a wheelie (press A twice) and you will fly over them. 

Power Boost  -  While racing press A, A quickly to do a wheelie and speed up. 

Race on the Moon  -  To race on the Moon you have to beat the WHOLE game including Florida. Beat Florida and you'll see your car get picked up by a helicopter and dropped into a rocket. Then it will show the rocket liftoff into space. You'll land on the Moon and here a man say, "We're cruisn' now!" Wait for the credits to end. When they're finished you'll see a tiny box that say "Moon expert". 

Secret Cars  -  Go to the practice mode and pick "practice championship". Beat the following tracks in less than the posted times to get the cars. Good Luck! 

Track Time Car
Australia 1:49 Surgeon 
China 1:14 Enforcer 
Egypt 1:07 Skool Bus
England 1:46 Bulldog 
France 2:15 Tommy
Germany 2:27 New York Taxi
Hawaii 3:47 Monsta 
Japan 2:48 Rocket 
Kenya 2:06 Conductor 
Mexico 1:46 Howler
New York 2:11 Grass Hopper
Russia 1.58 Rocket

There is also a secret car called the Speed Demon. To get it, you must finish gaining all of the points in the game's Champion mode. 

Secret Ramp in Germany  -  After you go through the town there will be a big turn. Then there will be a ramp. Pass it up and then pass a sign that says "80" then another ramp. Turn sharp to the left on the grass (the ramp is on the grass it is grass) start tapping "A" and you should perform a stunt. This helps a lot on Championship but it slows you down so I don't advise it on other modes. Good Luck! 

Shortcuts:-

Australia  -  A little while after the 1st checkpoint there will be a dirt path on the left. Take it, bust through the fence and get ready for another jump. 

Great Wall of China  -  When you see a lighthouse to the left drive towards it and take the path. Keep going straight. 

Hawaii  -  Near the beginning in Hawaii there are road signs telling you to turn. On the left side turn at the first sign, then immediately there will be another sign, ram into that sign and go straight and you will be on a shortcut. When you see a bunch of barrels on the left side on a turn without a curb, drive into the forest and you'll go through a shortcut and you'll jump over all the cars. 

Italy  -  Go to the Italy track in CRUISE THE WORLD mode. After you pass the 2nd or 3rd checkpoint you will drive down a tree-lined road. The second before you come out of that little alcove of trees you'll see a ramp to your left. Drive up that ramp to save some time. 

Kenya  -  Go to Kenya in Practice Cruise the World mode. After the third checkpoint, at the first right turn the road splits in two, but you have to veer right. You will be going straight at a ramp and then you will see an elephant right before you hit the jump. That's the shortcut! 

Mexico  -  In Mexico at the Aztec Temple, go into the brush to the left of the temple. There you will find a ramp and a shortcut. 

New York  -  In New York on the last two tunnels take any of the offramps on the side. 

Style Points  -  If you place in the top three championship races or pull off cool stunts while jumping ramps or other vehicles, you'll earn points to upgrade your ride. Check the chart below to see how to do tricks and how many trick points you'll earn if you do them right.

TRICK= Two wheels
MOVE= Double tap A while turning
POINTS= 0

TRICK= turbo
MOVE= double tap A
POINTS= 0

TRICK= braking drift
MOVE= press AB while turning
POINTS= 0

TRICK= jump flip
MOVE= turbo over ramp or opponent
POINTS= 1

TRICK= super heli
MOVE= do a braking drift over ramp
POINTS= 1

TRICK= mega flip
MOVE= go over ramp on two wheels
POINTS= 2 

Target Point Prizes - Here's the list of the target point prizes in Championship Mode.
8 Points: Power Level 2, Max Speed 160 
20 Points: Paint Job 
100 Points: Power Level 3, Max Speed 178 
150 Points: Two-Tone Paint Job 
500 Points: Power Level 4, Max Speed 189 
1,500 Points: Power Level 5, Max Speed 208 
9,999 Points: Speed Demon Car, Fastest in Game! 

Turbo Boost  -  When any race is about to start, you'll hear "Ready...Set...Go". As this is starting, hold down the accelerator just after you hear "Set". If your timing is just right, you will get a turbo boost to start the race. This trick may take practice, but well worth the time and effort. 

Windshield Splatter  -  Go to Mexico and all through the race have the in-car view on, Tap up-C once, then every once in a while you will see some bugs splatter on your windshield. This also works on some other tracks.