Unlock All Characters:
     At the main menu, press L2, R2, L2, R2, Up, Right, Down, Left, L1, R1.

Left Handed Players:
     When  selecting  a  character, hold SELECT + L2 and press X. Your
character will swing left handed.

Different sounds (Japanese version):
     At the main menu, press L1, R1, L2, R2, L1, R1, L2, R2. For every
time  you  enter  the  code, the sound effects for your character will
change.

Different balls (Japanese version):
     Just  before  hitting  the  ball,  While you see the swing meter,
press  L2  + R1 + Triangle. Change the balls shape by pressing Left or
Right.