Mario Kart 8 has a number of secrets awarded to players who accomplish special feats over the course of regular gameplay.
Ending
Beating the Special or Lightning Cups in third place or better on 150cc will let you watch the standard ending credits to Mario Kart 8. This can be done while playing in Multiplayer offline, so long as one player meets this requirement.
True Ending and Title Screen Change
Finish first in all cups on 150cc and you unlock the true ending credits, and a new title screen!
Gold Wii Wheel
You are awarded the Gold Wii Wheel by using Motion Controls in 50% or more of your last 100 races. Good luck...