Bonus boarders:
Attain an overall first place ranking in each difficulty level under competition
mode to unlock Ben, Nieno, Tok, and Boreth.

Bonus boards:
Finish in first place in the various races to unlock the XXX6, Top Gear Rally,
and Midway boards. Complete Stunt Challenge mode to unlock the Bucky board.

Master mode:
Beat the game on the first three difficulty levels to unlock Master Mode.

Twisted mode:
Beat the game on Master Mode to unlock Twisted Mode.

Mirror mode:
Beat the game on Twisted Mode to unlock Mirror Mode.

GameShark Codes

Have 50000 Stunt 	810829b8c350
Points	

Low Timer		811496000000
			811496020000

Extra Boards		81082a040003
			81082a06ffff

Extra Tracks		810829fcbffe

Extra Characters	810829f807ff 

Extra Difficulties	810829f00700

Thanks to Revolution readers LDK, Icepac555, Alex, and Frankenburger!