Win a gold cup in all races to unlock the Special Cup circuit.
Super Mario Kart tracks
Get 100 coins or more by the end of the cup to unlock extra tracks. Press L or R to view and play them. Once you get an "A" rank on every cup in every class you will unlock all of the original tracks from Super Mario Kart.
During a race, press Select to use the horn.
Control player selection screen
At the character selection screen, press L to shoot a green shell or press R to jump.
Once you have unlocked the SNES tracks in 150cc you will then be able to play through them in the Time Trail mode.
Unlock Classic SNES Tracks in 1P mode
If you want to race on the Classic SNES tracks that you play on in the single cart link mode while in 1P mode, all you need to do is complete each circuit with at least 125 coins collected and you will unlock the respective classic circuit.
To get extra tracks do any GP and get about 20 coins in each course or more, then at the end of all the courses when you get a trophy and they tell you your rank it adds up your coins if you've got enough you should have extra courses.
NOTE: If you do it to only number one in the GP you will only have extra courses for number one and you only get it for GP I think.