Decrease time warp:
Pause the game, press R2, L2, L1, R1, Left, Left, Left.

Invisibility:
Pause the game, press R1, L1, L1, R2, L1, L1, L2.

Lighter cars:
Pause the game, press L1, R1, R1, L2, R2, R2.

Increase time warp:
Pause the game, press R1, L1, L2, R2, Right, Right, Right.

No gravity:
Pause the game, press R1, R2, R1, R2, Up, Up, Up.

New Extra Cars:
Take Midnight Racing Club and save your cars the load them on Smuggler's Run and you will have extra cars!

Hidden Tunnels:
From where you start, drive over the sand dunes in the desert level until you are in plain view of the windmills. Drive over the hills and you will see a long dirt road.
Follow the road towards the 1st border check point. Keep driving until you pass the antique post office and a small group of hitchikers.
The end of the road should be near, find then end and take a right.
"4-Wheel" through the mirage pond until you see a oval tunnel.
Drive through that tunnel, and you will end up in a "ghost valley", then follow the dirt tracks until you get to another tunnel but do NOT go into it.
Instead, take a right up a steep hill. Once at the peak of that hill, go under the boulders located to your right, and follow the road that leads to a pit.
Drive off into the pit, and you'll find another hidden tunnel.
Go into the tunnel and you will now end up in another giant dark pit!
Go into the new pit and you'll notice ANOTHER hidden tunnel.
You can either drive into the tunnel and take a gigantic flying dive directly over the post office or you can come out the other end and be launched 50 feet into the air! This is where you can climb the side of the mountainous hill to get to the top of the desert mountains. Just use make sure you use the Hummer, since it can climb large objects.
Once your on top of the mountain, you will see that it is the highest point in the level, allowing you to see everything there is. You will also notice a black wall that is the end of the map! There are plenty of traps, so be careful! Enjoy the view!

On Top Of The World:
To get up to the edge of the map, find the archway that is at the top of the long stairs. Climb onto the cliff that it connects to and then drive over the arch and up the hill. Fllowing the path leads to a dead end with a spectacular jump if you climb the wall there. If you climb the wall on the left side of the hill that you just drove across via the arch, you are now on the highest elevation on the map. Slowly drive along the ridge until it broadens out. Once on top you can go nearly half way around the map, bounded by the jump/pit area on the southwest corner and the airfield on the west.