Play as Students:
Defeat the students you face in the Junior, Senior, Varsity, and Traveling Leagues to unlock them at the character selection screen.

Castle Court:
Play Luigi's Shooting Star mini-game and win both levels. When you get to the third level, beat the high score of 60. You can now play on the Castle Court in linked play and exhibition modes.

Tropics Court:
Play Baby Mario's Target Shot mini-game and win both levels. When you get to the third level, beat the high score of 60. You can now play on the Tropics Court in linked play and exhibition modes.

Jungle Court:
Play Donkey Kong's Banana Bunch mini-game and win both levels. When you get to the third level, beat the high score of 60. You can now play on the Jungle Court in linked play and exhibition modes.

Star Court:
Play Mario's Boo Blast mini-game, complete levels 1 and 2, and beat the high score to unlock the Star Court.

Nintendo 64 Characters:
Use the Transfer Pak with the Nintendo 64 version of Mario Tennis. Select "Transfer" at the main menu, then select "Game Pak Check". After it checks, an icon that says "Transfer N64 Characters" will appear. This will allow you to play as Bowser, Waluigi, Wario, and Yoshi.

Play as Peach:
Complete the game in Doubles.

Play as Mario:
Complete the game in Singles.

More Mini-Games:
Unlock the Nintendo 64 characters, then check the mini-game screen. More entries should be available. To unlock even more mini-games, win the Junior, Senior, Varsity, and Traveling Leagues in Singles and Doubles.

Get All Levels:
At the main menu press A, A, B, B, Up, Up.

Thanks to Revolution reader Blade and ryan stubbs!