Unlockables

Alternate Title Screen and Ending Screen

Courses

Mirror mode

Unlockable Bikes
After meeting the requirements, some unlockables may require you to back out of your save file and reselect it. Note: In different countries, the bikes have different names but the unlocking method is the same.

Characters

Unlockable Karts
After meeting the requirements, some unlockables may require you to back out of your save file and reselect it. Note: In different countries, the karts have different names but the unlocking methods are the same.

Have a star by your nickname
Playing extraordinarily well in Grand Prix races can produce a Star Ranking upto 3 at the end results instead of the standard "C, B or A."

Collecting these stars in all Grand Prix's adds a Star Icon next to your name in WiFi Races and on the Results Screen for your opponents to see.

(Note: Say, if you get a Three star rating in every Grand Prix except one which you got a One star rating in. You would get a One starred nickname until you manage to upgrade the rank.)

Golden Wii Wheel

Karts and Bikes versions of 50cc and 100cc
You can unlock Karts and Bikes versions of 50cc and 100cc(like 150cc, but slower) by completing all the cups in that CC.

To activate these versions, press the + button on the Speed Select screen.

Secrets

Quick Start
When the countdown starts wait till the 2 starts to disappear and then press and hold the 2 button. So the 2 must still be visible but disappearing.
This will give you a boost at the start of the race.

Boost when recovering from a fall
When you fall into something, such as water or falling from rainbow road, when you land, if you hit 2 at the moment you hit the ground, you get a temporary boost.

POW Block Immunity
When you see the POW block on your screen and its about to hit you, just do the function you would normally use for a trick to create a shockwave for youself to become immune to the spin out the POW block creates (Control panel on GCN controller, Shaking Wii wheel, etc.). This can be done on both bikes and carts. Even though you don't loose speed or spin out, you still will loose the items you were carrying.

Change Mii Racer weight class
Go into the Mii Channel and change your Mii drivers weight, which will have an in game impact causing you to change weight class and have different cars.

Glitches

More than one person can be the same character.
First, make P1 choose the character you both want to be. Then unselect the character and move off of it. Move P2 over, and select the character. Go back one screen, then go into the character select again. You should both be on the same character. Both of you select the character. Do whatever you want from there on.

Note: In the character select screen and options, whenever one of you does an animation, both of you will. Also, if P2 chooses a different kind of vehicle than P1, P1 will look strange. Don't worry, this won't actually happen in the game.

I haven't tested this with P3 or P4 yet, but in theory, it should work.