Add Extra Laps
When your car is about to cross the line after completing a lap, repeatedly pause and unpause your game (if you have a controller with a Slow-Mo function, enable it here) as it goes over the line. If your timing is right, your lap counter will increase every time you unpause your game.
Infinite Continues
Power the console off, press and hold A + B on controller 1, then power it on again to have infinite continues.