Invert track:
Press R + L+ Down at the track selection screen. 

All cars:
Enter CARTOON as a name. Note: Enter CARNIVAL as a name in the European
version. 

All tracks:
Enter TRACTION as a name. Note: Enter TRACKER as a name in the European
version. 

All weapons: 
Enter YOY as a name. Press L + R to change weapons. Note: Enter SADIST as a
name in the European version. 

Tiny mode:
Enter MAGGOT as a name.

Change cars:
Enter CHANCER as a name.

Enable UFO:
Enter YUEFO as a name.

Hi-Fi mode:
Enter FLYBOY as a name to increase the car details.. 

Edit progress table:
Enter GOATY as a name.  Yeeah, that's right.