To get extra cars & tracks, beat a championship race (bronze, silver, gold, or platinum, it doesen't matter) finishing first in points standing. In bronze you unlock slower cars but in platinum you will unlock faster cars. With tracks you unlock harder tracks in gold and platinum, but in bronze and silver you unlock easier tracks.
Jump in Multiplayer
Just press C-Up during a multiplayer game
Mirror Tracks
To unlock mirror courses, beat Time Trial Challenge times on all reverse tracks in each circuit.
Movement Hints
C-Up will get you unstuck, out of water, etc.
R will flip you right side up when flipped over
Reverse Tracks
To unlock reverse courses, beat Time Trial Challenge times on all (forward) tracks in each circuit.
Reverse-Mirror Tracks
To unlock reverse-mirror courses, beat Time Trial Challenge times on all mirror tracks in each circuit.
All Cars and Tracks...