Luigi Circuit for sure!
One of the ones that haven't been unlocked like Peach Gardens, Jungle Zoo etc.
Play a Mario Kart game that has the track Yoshi Valley in it, or build your own Mario Kart
For me, Luigi64thegr8 is theawesome, They are all easy. The easiest, is like Mario Kart or Super Mario USA.
The easiest way to get Rosalina in Mario Kart Wii is to buy Super Mario Galaxy and make a save file in the game. The next time you play Mario Kart Wii, you should have unlocked Rosalina.
Mario Raceway
Luigi Circuit
Birdo cannot be unlocked in Mario Kart DS. Sorry :(. She is in Mario Kart: Double Dash!! and Mario Kart Wii though. She is only unlockable in the latter. The easiest way to unlock her in Mario Kart Wii is by completing sixteen races in Time Trials.
a race track on mario kart wii
yoshi valley
Unlock 1 Expert Staff Ghost. The easiest track for this is probably Grumble Volcano due to a massive shortcut at the start of the race.
Luigi Curcuit
the ones all ready there