To get ghost racer simply choose any track and don't touch the sides, and race the ghost rider.
Enter time trials and hover the cursor beside Mushroom cup then press L,R,L,R,L,L,R,R,A. Special cup will then appear below star cup.
When at the character select screen (only in racing) hold Y and push A and your chacter will become small. This will not effect speed whatsoever, it will only make it so your opponents can run you over to slow you down. Hold Y and push A to make yourself big again.
Hold Y, then push A - Shrink your racer
Erase all ghost times
To erase all ghost data press L,R,Y,A and tis will take you to reset mode.
This is quite a cool secret, all you do is hold Y and then press A on the character selection screen. Anotehr way is to highlight the character you want to be and press A,B,X,Y at the same time.