All Tune-ups:
At the title screen, hold L+B and press up.

All Tracks:
At the title screen, hold L+B and press right.

All Extras:
At the title screen, hold L+B and press down.

All Cars:
At the title screen, hold L+B and press left.

Expert championship difficulty:
Beat championship mode with a Gold rank on all tracks under normal difficulty.

Thanks to Revolution readers Carlos and Abe!