To unlock all bonuses, at the main menu Hold L1+L2+R1+R2+R3 and push up on the analog sticks, then also hold L3 and push down.
Big Head mode:
To unlock Big Head mode, pause the game and Hold L1+L2+R1+R2+R3 and right on the analog sticks, then also hold L3 while pushing left.
Single-player mirror mode:
To unlock Single player mirror mode, pause the game and hold L1, L2, R1, R2, L3 and right on the analog sticks, then also hold R3 and push left.
Short-cut on "Grizzly" track:
On the Grizzly track after the jump at the end of the lap, look to your left and you will see a few barrels. Drive through them to access a shortcut.
To clean your screen if it becomes covered in mud, tap Triangle twice.
While you are approaching the finish line have your boost meter almost all the way full and turn on the boost. Continue to hold the boost as your car heats up. Your car will quickly explode propelling you across the finish line at a very high speed.
Perform a burnout:
To perform a burnout, hold L2+R2 and press circle. Then, release circle.
Blind your opponent:
You can blind your opponent while driving in mud by driving in front of them and making your tires kick mud into your opponents screen.
To make yourself go faster while driving any vehicle, let you boost charge and repeatedly tap X. This will make any vehicle go faster and not use boost.
When on any track with big or long jumps, hold the D-pad in any direction and your vehicle should do a flip in that direction.
Easily controlled vehicles:
The dirtbike and quad are the two most controllable vehicles due to their ability to easily ride along the walls.
Runcorn easter egg:
When you see the front of your car or bike before a race, notice the banner that reads "Free The Runcorn". Runcorn is the town in the U.K. where the game was created.