Mario Kart: Super Circuit (GBA)
Erase all data
To erase all saved data at once, press and hold Start + L + R + B during the "GAME BOY" boot-up screen. Keep holding the buttons, and a screen should come up where you'll have to answer yes or no to the "OK to erase all saved data?" question.
On the "Choose a Character" screen, you can make the selected (rotating) racer shoot a Green Shell, jump, and honk his or her horn. Just press L, R, and Select, respectively. If "Random" is selected, you won't be able to use the horn.
Unlock Special Cups
Get the gold trophy in each of the four other cups (Mushroom, Flower, Lightning, Star) to unlock the Special Cup. You'll have to do this separately for each engine class -- 50cc, 100cc, and 150cc.
Unlock Special Cup in Time Trial mode
Unlocking the Special Cup in the 150cc class will also unlock the Special Cup in Time Trial mode.
Unlock Super Mario Kart tracks
All 20 tracks from Super Mario Kart are hidden in MK: Super Circuit. To unlock them, you'll need to have the Special Cup unlocked first. Then, after you get the gold trophy in the Special Cup, play each cup again. Finish with 100 Coins or more to unlock four SMK tracks. After you've unlocked some tracks, press L or R on the "Choose a Cup" screen to switch between the MKSC tracks and SMK ("extra") tracks. You can only unlock four extra tracks per cup, and you'll have to do this separately for each engine class -- 50cc, 100cc, and 150cc.
Unlock Super Mario Kart tracks in Time Trial mode
Unlocking the extra tracks for a cup in the 150cc class will also unlock the extra tracks for the same cup in Time Trial mode.