Extra Characters and Canyon Court
At the "Smart Dog" logo screen press: up, down, left, right, Start. If you do this correctly you should hear a guy cheer. This should give you access to the extra players and the Canyon court which is normally not available in Smash Tennis mode.

Fire Backhand
For a more powerful backhand, hold R1+R2 and press X, Square, Circle, Square, X, Triangle. Release R1+R2 then press Select.