Turbo boost hint:
Start a race and, as the lights count down, wait until you see the yellow monkey. When he appears, press and hold X to start the race with a turbo boost.
Unlock everything code:
To unlock every race and racer, go to the title screen and press the following: CIRCLE, TRIANGLE, CIRCLE, SQUARE, X, SQUARE, X, TRIANGLE, TRIANGLE, X, SQUARE, CIRCLE. Afterward, go to the Challenge screen and press X to choose a challenge. You'll have access to all riders and every race.