Gold-Colored Steering Wheel
If you utilize the steering wheel that comes with the game extensively (or even exclusively) instead of using the Wii controller or a GameCube controller, you'll find a gold-colored steering wheel icon next to your name when you play online.

Small Karts
To unlock the small karts listed below, fulfill the requirements listed.
Blue Falcon - Win the Mirror Lightning Cup
Cheep Charger - Earn at least a one star ranking in all 50cc Retro Cup races
Rally Romper - In Time Trial mode, unlock the expert staff ghost data record

Alternate Title Screen
To unlock an alternate title screen that will be shown when the game is started up, win each and every cup available in the game.

Your Name In Stars - Mario Kart (Wii)
[Submitted by: Toebin ]
In the online rankings you can get from one to three stars next to your Nickname.

Description: ONE STAR:
How To: Get a one star ranking in all Grand Prix

Description: TWO STARS:
How To: Get a two star ranking in all Grand Prix

Description: THREE STARS:
How To: Get a three star ranking in all Grand Prix

Victory Photos
Description: unlock both victory photos available in the game (which is basically a shot of your Mii with various characters), you have to fulfill two requirements. To unlock the first photo (which is of your Mii with Mario and Peach), earn a trophy in every cup for each class available. To unlock the second photo (which is of your Mii with each racer in the game), win each and every Grand Prix cup available in the game.

Large Bikes
To unlock the large bikes listed below, fulfill the requirements.
Phantom - Win the Mirror Cup
Torpedo - Unlock twelve Expert Staff ghost data in Time Trial mode
Twinkle Star - Win the 100cc Star Cup
Addition by mr caspartt

How to get Expert Ghosts - Mario Kart (Wii)
[Submitted by: Toebin ]
Beat a normal course ghost by seven seconds or faster.

Characters
Description: unlock the characters listed below, fulfill the requirements listed.

Baby Daisy	Earn at least a one star ranking in each 50cc Wii Grand Prix cup races
Baby Luigi	Unlock eight Expert Staff ghost data in Time Trial mode
Birdo	Defeat 250 people in online racing, or play Time Trial mode on sixteen different courses
Bowser Jr.	Earn at least a one star ranking in each 100cc Retro Grand Prix cup races
Daisy	Win the 150cc Special Cup
Diddy Kong	Win the 50cc Lightning Cup
Dry Bones	Win the 100cc Leaf Cup
Dry Bowser	Earn at least a one star ranking in each 150cc Wii Grand Prix cups
Funky Kong	Unlock four Expert Staff ghost data in Time Trial mode
King Boo	Win the 50cc Star Cup
Mii	To play as the Mii associated with your profile, win the 100cc Special Cup
Rosalina	Have a Super Mario Galaxy save on your Wii
Toadette	Play Time Trial Mode on all thirty-two courses

Avoiding POWs
Getting attacked with a POW can be a bummer, but if you flick upward your steering wheel or Wii remote, you can avoid it. You will spin around, and you will lose your currently-equipped item, but you won't slow down and will avoid the devastating result of being struck with a POW.

Mario Kart (Wii) Golden Wheel
[Submitted by: Toebin ]
To get a Golden Wheel next to your name online just use the Wii wheel extensively in all modes.

Do that and you'll see your new gem (wheel) beside your name when you go online.

Medium Karts
To unlock the medium karts listed below, fulfill the requirements listed.
Royal Racer - Win the 150cc Leaf Cup
Turbo Blooper - Win the 50cc Leaf Cup

Medium Bikes
Description: unlock the medium bikes listed below, fulfill the requirements listed.
Dolphin Dasher - Win the Mirror Star Cup
Nitrocycle - Attain at least a one star ranking in all 100cc Wii Grand Prix cup races
Rapide - Win the 100cc Lightning Cup

Bikes on 50cc Cup Races
To unlock the ability to drive bikes in 50cc cup races, earn all of the trophies on each 50cc cup race.

Mirror Class
To unlock the Mirror Class, win all of the available 150cc Cups in the game.

Quick Recovery -Mario Kart Wii (WII)
[Submitted by: Toebin ]
If you find yourself off the track you will be brought back by the game but forced to start over. To make up for lost time just hold down the Accel button just as you are dropped back on the track. You will notice a boost to your speed.

Small Bikes
To unlock the small bikes listed below, fulfill the requirements.
Bubble Bike - Win the Mirror Leaf Cup
Magikruiser - Play Time Trial Mode on any eight courses
Quacker - Win the 150cc Star Cup
Addition from mr caspartt

Cups
Description: unlock the cups listed below, fulfill the requirements listed. Keep in mind that the requirements are the same for each CC type on the circuits listed.
Leaf Cup - Earn trophies in the Shell Cup and Banana Cup
Lightning Cup - Earn a trophy in Leaf Cup
Special Cup - Earn a trophy in Star Cup
Star Cup - Earn trophies in the Mushroom Cup and Flower Cup

Large Karts
Description: unlock the large karts listed below, fulfill the requirements.
Aero Glider - Earn at least a one star ranking in each 150cc Retro Grand Prix races
Dragonetti - Win the 150cc Lightning Cup
Piranha Prowler - Win the 50cc Special Cup
Addition from mr caspartt

Karts on 100cc Cup Races
Description: unlock the ability to drive karts in 100cc cup races
How to unlock: earn all of the trophies on each 100cc cup race.