Hover car cheat
For hover car cheat turn on your GBA and when you get to the Title page select new game and enter hovercar as your name.

Tiny cars
To enter tiny car cheats follow the steps above but enter your name as rcracers.

Low gravity cheats
Follow the instructions on the hover car cheats but enter your name as moonwalk.

Turn all road obstacles into cows
Follow the instuctions on the hover car cheats but enter your name as rogueops.