Codes acquired from codes.ign.com
Mario Tennis

Unlock Hidden Courts
To unlock the hidden courts in Mario Tennis, you'll need to play and beat 
several stages in the game. The courts, and how you open them, are 
explained below: 
Warehouse Court: Beat all of the Mario characters with a Mario character 
to get this court. 
Castle Court: Get 60 points or higher in the third level of Shooting Star.
Jungle Court: Get 60 points or higher in the third level of Banana Bunch. 
Tropics Court: Get 60 points or higher in the third level of Target Shot. 
Star Court: Get all of the medals on the Clear Status screen.


Unlock Mario
To get the ultimate tennis baller, you'll need to finish the entire Mario 
Tour in Singles.

Unlock Peach
To unlock a playable Peach, beat the entire Mario Tour in Singles.

Unlock Special Rackets
When you become a member of the Travelling Team, go visit the man hanging out 
near the pond in the upper right corner of the Training Court. 

This cheeky lad will challenge you to a racket-swinging contest. You'll have 10 
seconds to swing your racket. If you are able to swing it 100 times in this time 
frame, you'll earn the Silver Racket. If you can (and this is near impossible) 
swing the racket 150 times, you'll get the Gold Racket.

Use the Transfer Pak to Unlock Characters
If you have the N64 and GBC versions of Mario Tennis, you can use the Transfer 
Pak to unlock new characters. In order to transfer certain players, you will 
have to unlock them first in the N64 version of the game. Then, pop the GBC cart 
into the Transfer Pak and download your new characters!