Mickey's Speedway USA

Quick start:
Immediately press A as the third light turns on during the pre-race countdown.

Negative coins:
Win a gold or platinum on the mirrored Traffic Troubles.

Digital speedometer:
Win a gold or platinum cup on the mirrored Motorway Mania.

Tiny racers:
Win a gold or platinum cup on the mirrored Freeway Phobia.

Mirror mode:
Win at least an gold in every tournament, including Victory Vehicles and Frantic Finale.
Information in this section was contributed by Sohail.Chaudhry.

Dewey:
Win a gold cup in all circuits under the amateur difficulty setting to unlock Dewey.

Louie:
Win a gold cup in all circuits under the intermediate difficulty setting to unlock Louie.

Huey:
Use a Transfer Pak with the Game Boy Color version of the game to unlock Huey.

Ludwig von Drake:
Win a platinum cup under the professional difficulty setting in the Frantic Finale tournament.

Rainbow Goo:
Win a gold or platinum cup in the first three circuits under the amateur difficulty setting.

Heckle button:
Win a gold or platinum cup in the first three circuits under the amateur and intermediate difficulty settings.

Unlimited retries:
Win a gold or platinum cup in the first three circuits under the amateur, intermediate, and professional difficulty settings.

Disable boost pads:
Win a gold or platinum cup in mirror mode on the Victory Vehicles circuit:
Information in this section was contributed by Patrick Schiess.

Jukebox:
Win a platinum cup on mirror mode in the Frantic Finale tournament.

Victory Vehicles circuit:
Win the first three circuits under the professional difficulty setting to unlock the Yellowstone, Everglades, Malibu, and Washington, D.C. races.

Frantic Finale tournament:
Collect all four car parts.

Hint: Chicago: Car part:
Do not go up where the track splits after you go down the steep drops. Instead, make a left but do not go right after that. Keep going straight to discover this car part.
Information in this section was contributed by KWGrandpaGoofy.

Hint: Indianapolis: Car part:
Make a sharp left turn immediately after you get out of the tunnel with the green "GO" sign across it. There is a little hole there. You will find a spark plug in it.
Information in this section was contributed by ChrissyBear17.

Hint: Malibu: part:
When you start, go straight to where the water meets the mountains. You will be granted the secret passage there. Go in there and find the car part.
Information in this section was contributed by KWGrandpaGoofy.

Hint: Philadelphia: Car part:
When the track turns into a ship, keep going straight ahead. You will be granted the part after you make a sharp left turn to get back on course.
Information in this section was contributed by KWGrandpaGoofy.