These codes are entered under the Extras menu.

All cars and 1 million credits:
Enter GIEVEPIX as a code.

All tracks:
Enter GIVEALL as a code.

Pimpster:
Enter RUTTO as a code.

Flatmobile:
Enter WOTKINS as a code.

Ice cream truck:
Enter RAIDERS as a code.

School bus:
Enter GIEVCARPLZ as a code.

Rocket:
Enter KALJAKOPPA as a code.