These include the Mii Channel, for example, where you go to create your customized Mii avatar characters.
While choosing your Mii character, hold one of the four directions on the directional pad on the Wii Remote.
Eventually, the Mii character on the screen will fall asleep and the background music will change.
Perhaps the most interesting unlockable characters are the Mii avatars.
To play as your Mii in the first outfit, get first place in the 100cc Special Cup grand prix series.