Hidden Characters
After beating the Mirror Track (stages 1-4)with Skier and Boarder, 
watch the credits and another screen will pop up that says you can 
select the hidden characters. the bear, robot, and the disco guy)

Mirror Stages
To get Mirror Stages you must beat the regular Stages 1-4 the after 
the credits a screen pops up and says now you can select the mirror 
stages by pressing Z. This works in Time Attack, 2 player, and 
Championship modes.

All Levels
To get all level cheat code. First go to the screen where you choose 
you're level to board on then hold the top "L" button on top and 
hold on to it then you must press "z" button quickly and press the 
"top C button" then press c down then you should have the code