Faster vehicles:)
At title screen, push A, B, L, A(2), R, B 

Alternate vehicles:)
At title screen, push A(2), B, A, L, R, A, B, L

Slower opponents:)
At title screen, push A, R, L, B, A(2), B, L

All tracks:)
At title screen, push B, L, B, A, R, B(2), A(2)

Password screen:)
At machine selection screen during Grand Prix mode, push L, R, Start, R, L, Select

In-game reset:)
While playing push Select + Start + A + B .

Demonstration mode:)
At title screen, push Select

Unlock more demonstration cars and modes:)

Win all of the tracks or have unlocked all ten cars this will unlock more modes and demonstration cars.

Delete your saved games:)
Press and hold L + R and turn ON the Game Boy Advance. Pick "Yes" at the prompt screen to delete your game saves.

Obtain your ranking password:)
At main menu choose Ranking . Select the desired series, game, and course. Then, hold L and push Up, Right(2), B, Left, B, Right, A at the course ranking screen.

Unlock Silver Thunder:)
Beat the Queen tracks in expert difficulty mode.

Unlock Stingray:)
Beat the Pawn, Knight and Bishop tracks in expert difficulty mode.

Unlock Sly Joker:)
Beat the Pawn, Knight and Bishop tracks in standard difficulty mode.

Unlock Falcon Mk.II:)
Beat any set of tracks in master difficulty mode.

Unlock Fighting Comet:)
Beat the Pawn, Knight, Bishop and Queen tracks in master difficulty mode.

Unlock Silence:) Open C:)
This becomes unlocked when you play a linked game with only one cartridge.

Unlock Master difficulty setting:)
Beat the Pawn, Knight, or Bishop series under expert difficulty in grand prix mode to unlock master difficulty for that specific series.

Unlock Queen circuit:)
Beat the Pawn, Knight, and Bishop circuits in expert difficulty mode.