Air Boarder 64 was not released in the United States.
Unlock all four bonus characters. Then, press Up(2), Down(2), Left, Right, Left, Right, B, A at the board selection screen.
The following four bonus boards will be available
Ika-Chu (Double jumps)
Father (Longer air time)
J-B (Press A + B for turbo jump)
J-Arm: (Extra turbos)