Instructions
Unlock the secret tracks by completing all of the circuits in the game at any difficulty. This means you must get at least third place in the Banana Cup, Shell Cup, Flower Cup, Mushroom Cup, Leaf Cup, and Star Cup. Once you have unlocked the Special Cup, you have unlocked all of the tracks.
Get a rating of one-star in all of the circuits at every difficulty to unlock the following characters: Baby Daisy, Bowser Jr., and Dry Bowser. This should also unlock Daisy, Dry Bones, and Mii (Outfit A).
Play all the courses in Time Trial mode to unlock more characters. You will unlock Birdo after 16 courses, and Toadette after 32 courses.
Unlock expert staff ghost data in all 32 tracks to unlock more characters. You unlock Funky Kong after getting four ghosts, Baby Luigi after getting eight ghosts, and Mii (Outfit B) after getting 32.
Unlock the last secret character, Rosalina, by having a save file from "Super Mario Galaxy" on your Wii and going through 50 races.