The following characters can be unlocked in mariokart wii by doing the corresponding tasks:

King Boo - Win the 50cc star cup

Diddy Kong - complete all the 50cc cups

Dry bones - Win the 100cc leaf cup

Mii (outfit A) - Win 100cc special cup

Daisy - Win 150cc special cup

Baby Daisy - Get 1 star grade on all 50cc Wii cups (mushroom, flower, star and special)

Bowser Jr - Get 1 star grade in all 100cc retro cups (shell, banana, leaf and lightning

Dry bowser - 1 star grade in all 150cc wii cups

Funky Kong - unlock 4 expert staff ghost data on time trials (beat Nintendo staff record)

Baby Luigi - unlock 8 expert staff ghost data records

Birdo - get a time trial record for 16/32 courses

Toadette - get a time trial record on all 32 courses

Rosalina - Have a Super Mario Galaxy Saved game on your wii memory and you will unlock her after a few grand prixs