Play as Petey Piranha:
In doubles, beat both Special Cup tournaments.

Play as Paratroopa:
In doubles, beat the Normal Cup tournaments.

Star Cup:
Beat the Normal Cup tournaments.

Max difficulty:
Beat the Star Cup tournaments.

Unlock Bowser's court:
In singles, beat the Special Cup's Fire Tournament.

Mario Brothers' court:
In doubles, beat the Special Cup's Thunder Tournament.

Event mode:
At the title screen press Start while holding the Z button.