Hold R while selecting a character to make him/her contain handicaps with have more power.

Left-Handed Characters

Hold L while selecting a character to make him/her left-handed.

Special Cups

Go through all the Tournament Cups with all the characters (including Shy Guy and Donkey Kong Jr.) to unlock the Special Cups, which are much harder than the previous one. Press R while selecting a character to access the Special Tournament in the menu.

Extra Players

Shy Guy: Win the Star Cup/Singles Tournament.
Donkey Kong Jr.: Win the Star Cup/Doubles Tournament.

Extra Courts

Donkey Kong Court: Win the Mushroom Cup/Singles Tournament as Donkey Kong.
Yoshi and Baby Mario Court: Win the Mushroom Cup/Singles Tournament as Yoshi.
Mario and Luigi Court: Win Mushroom Cup/Singles Tournament as Mario.
Mario Brothers Court 2: Win all the Doubles Tournament Cups as Mario.
Mario/Luigi Castle Court: Win the Star Cup Doubles and Singles Tournament as Mario.
Wario Brothers Court: Go through all the cups for Double Tournament with Wario.
Piranha Plant Court: Complete any court (perfect, all 50 balls) in the Piranha Plant Challenge with any character.