Unlock and race as Huey
If you own the Gameboy version of this game or if you have access to the game AND access to a Transfer Pak you can unlock Huey. Just place the Gameboy version of the game into the Transfer Pak, use the transfer pack in your N64 controller and voila, Huey is unlocked.

Unlock and race as Dewey
To unlog Dewey, you must win all circuits on the Amateur difficulty setting.

Unlock and race as Louie
Place first in each of the first three circuits at the Intermediate difficulty level.

Quick start
Press A as the third light turns on during the pre-race countdown.

Unlock Victory Vehicles Circuit
Win each of the first three circuits at the Professional difficulty level to unlock the Victory Vehicles circuit of Yellowstone, Everglades, Malibu and Washington, D.C.

Frantic Freeway circuit
Collect all four race car parts to unlock the Frantic Freeway circuit.

Get All Car Parts
Indianapolis - In that level go through the first tunnel. At the end of the tunnel make a sharp left. The part is kind of in indented on the wall.
Philadelphia - When the track turns into a ship keep going straight. You will get the part after make a sharp turn left to get back on course. Chicago - When you're on this track you have to know where you're going. Don't go up where the track splits after you go down the steep drops. Instead, make a left but don't go right after that - keep going straight to find the third car part. Malibu - When you start go straight to where the water meets the mountains and find the secret passage there. Go in and find the last car part. Hint - You can quit right after and still have the car part.

Infinite retries
Win a gold or platinum cup in the first three tournaments on the professional difficulty setting.

Rainbow Goo
Win a gold or platinum cup in the first three tournaments on the amateur difficulty setting.

Heckle button
Win a gold or platinum cup in the first three tournaments on the intermediate difficulty setting.

Digital speedometer
Win a gold or platinum cup on mirror mode in the Motorway Mania tournament.

Tiny racers
Win a gold or platinum cup on mirror mode in the Freeway Phobia tournament.

Disable boost pads
Win a gold or platinum cup on mirror mode in the Victory Vehicles tournament.

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

Negative coins
Win a gold or platinum cup on mirror mode in the Traffic Troubles tournament.