Complete every single cup with a first place unlocks new alternate Title Screens and unlocks the true ending credits.
Banana Cup
Successfully complete the Shell Cup.
Bonus characters
Get a gold trophy by finishing first place in any cup and engine class in Grand Prix mode to unlock one of the following characters thirteen characters at random. Note: Once a character has been unlocked on a particular track and CC class, you will not be able to unlock another new character the exact same way.
Baby Rosalina
Gold Peach
Iggy
Lakitu
Larry
Lemmy
Ludwig
Metal Mario
Morton
Rosalina
Roy
Toadette
Wendy
Bonus parts
Collect the indicated number of coins to unlock body, glider, and wheel parts to customize your vehicles. You will unlock a new part for every 50 coins earned; after earning 1,000 coins, the intervals increase to 100 coins for each unlockable part. Coins can be earned in Grand Prix, VS mode, and online races.
Collect 10,000 Coins (Multiplayer gives players the total amount of Coins collected by all players during each race).
Gold Handle
Use the Wii Handle for 50% of your most recent 100 stages.
Gold Standard Kart
Get at least one star (54 points or more) on every 150cc and Mirror Mode race.
Gold Tires
Beat all of the Time Trials Staff Ghosts.
Grand Prix stars
Earn the indicated number of points in Grand Prix mode to be rewarded with the corresponding number of stars.
1 star: Earn 54 or 55 points (two 1st and two 2nd place finishes or three 1st and one 3rd place finish)
2 stars: Earn 57 points (three 1st place finishes and one 2nd place finish)
3 stars: Earn 60 points (four 1st place finishes)
Leaf Cup
Successfully complete the Banana Cup.
Lightning Cup
Successfully complete the Leaf Cup.
Manually choosing the season on the Animal Crossing DLC Track
If you hold a specific button while selecting the Animal Crossing stage with A, you can actually choose what season you play on. These are: L for Spring R for Summer zL for Autumn zR for Winter.
Miiverse Stamps
There are 62 Unlockable Miiverse Stamps in Mario Kart 8: 30 character stamps and 32 track stamps.
Stamp
How to unlock
Character Stamp
Finish 1st in any Grand Prix with that character.
Track/Course Stamp
Beat the normal staff ghost time for track.
Mirror Mode
Earn a Gold Trophy for every Cup in the 150cc difficulty to unlock Mirror Mode. This mode lets you race in reversed versions of the existing Cups.
Other racers
In Grand Prix mode, get first place on any Cup at any CCs and it will give one of the 14 Characters at random however you can not repeat the same cup with the same CCs.
Quick start
When "3" appears during the pre-race countdown, hold [Accelerate].
Racers
In Grand Prix mode, get first place on any Cup in any engine class and you will be given one of 13 additional characters at random - however you can not repeat the same cup with the same engine class to earn more characters. The one exception to the above rule is the Mii character, the 14th unlockable character. He will be unlocked when you obtain first place in every Cup of a single engine class.
Rocket boost
Press the accelerator button when "2" appears and hold it down for a rocket boost at the start of the race.
Special Cup
Successfully complete the Star Cup.
Star Cup
Successfully complete the Flower Cup.
Toad Harbor
Normally at Toad Harbor you will get knocked out by the trams when you drive into them by accident. However if you are flying after jumping from a ram or booster it is possible to use the roofs of the trams to your advantage. If you land on the trams roof you will bounce off it undamaged and if you can perform a flip as you are bounced off you can gain some extra boost.
Totaka's secret song
Lower the music volume, and go to a race that features Yoshi spectators in the crowd. They can be found at Donut Plains 3, Sweet Sweet Island, and the Yoshi Circuit in DLC 1. Eventually one of them will hum Totaka's secret eight note song instead of their normal random grunt. Kazumi Totaka's is Nintendo sound designer and has placed the same secret song in other games, such as Mario Paint and Animal Crossing.
Using trams as a speed boost in Toad Harbor
The trams in Toad Harbor usually knock you out when you drive into them by accident, but if you are flying after jumping from a ramp or booster, you can actually use the roofs of the trams to your advantage. While in the air, land on a tram's roof to bounce off it undamaged. For an extra boost, perform a flip.
Vehicle Parts
Vehicle parts are unlocked at random every time you get 50 coins. Once you reach 1000 coins total kart parts are unlocked every 100 coins. The only exceptions to this rule are the gold parts.