Get Mystery Rider
To get Mystery Rider beat all the tracks on easy mode then he will sometimes be choosable. P.S the Mystery Rider is Plankton from the T.V. show Spongebob Squarepants.
Hidden Racers
If you beat the game 3 times you can get 5 new racers. You can get Phli and Lil from the rugrats, Sid from Hey Arnold, Doug from Doug and Otto from Rocket Power.
Turbo Boost
At the beginning of a race, when it's on 3, hold down the up button on your keyboard and you should get a boost.