Overhead View
Press Up, Down, L, R, Select at the title screen. Enable the new "Mystery Mode" selection at the bottom of the options screen.

Drive Boss Car
Press L, R, Up at the title screen. A rumbling sound will confirm correct code entry. Start a normal game. After reaching the Boss, you will be driving his car.

Alternate Car Colors
Highlight a car at the selection screen then press A + Select + Start. You will now be able to choose a color for your car.