Boost your character's stats
To have your character specialize in their technique, beat the singles tournament (mushroom, flower, and star cups) with them. Also their abilities (i.e. serving the ball) will also be raised a little. Such as bowser, with a star on i have seen him serve the ball up to 130 mph!! To put a star on him on the select character screen hold R and press A. Remember this only works if you beat the singles tournament with a character.

Unlock Donkey Kong Court
Unlock this court by winning the Mushroom Cup/Singles Tournament with Donkey Kong.

Unlock Yoshi and Baby Mario court
You can get this court if you win the Mushroom Cup/Singles Tournament using Yoshi.

Unlock Shy Guy
Unlock Shy Guy by winning the Star Cup/Singles Tournament.

Unlock Donkey Kong Jr.
You can unlock Donkey Kong Jr. by winning the Star Cup/Doubles Tournament.

Mario Bros. Court
Unlock Mario Brothers Court by winning the mushroom cup/singles tournament with Mario.

2nd Mario Bros. Court
Unlock the 2nd Mario Bros. court by winning the mushroom cup/doubles tournament with Mario

Nintendo Power Cup Code
At the main menu, got to special games and select ring tournament. Type in this code with pressing A after each letter/#-J6M9PQJ6U. You can now play as DKJR against Bowser for three minutes trying to earn as many rings as possible.

2 Codes
Here are two fun "codes" to use:
1. Hold L while choosing a character to be left-handed.
2. Hold R while choosing a character to have handicaps.

New Court In One-Player Mode
Go to the game with the tennis ball spitting piranha plants, get a perfect score (all the balls), and go back after exiting. A new court should be there for the piranha plant game.

New Computer Opponent Level
After getting the "Star Tournament", use all 16 characters and beat all cups (rainbow, moonlight, planet) to get the "MAX" difficulty level of your computer opponents. Use it, if you think the computer players don't offer much of a challenge... But at that level, they're going to kill you.

New Tournament
Use all 16 characters and beat star cup, doubles and singles. When that's done, go look at the characters and use the "star" near them to enter the "Star Tournament".

Birdo and Yoshi court
Beat star cup doubles mode with Birdo.

Wario and Waluigi court
Beat every cup with Wario doubles tournament

Unlock Rainbow, Moon, Planet Cup
To unlock the Rainbow Cup you will have to beat the Star Cup with all characters. Then when you go to the character select screen you will need to hold R when you select your character. The game menu will come up with a star on top of the cup in tournament mode. This mode is the same as the normal mode except that it has different names. this is the order; Rainbow Cup, Moon Cup, Planet Cup.

Ring shot easy win
Get 40 points then just stand there and let every ball go past you until your opponent gets 40 points and ties with you then you get as many rings as you can and win the next point then let next ball pass and repeat those steps so you keep tying.

Play Tournament Passwords
Put in these codes at the code screen to play a tournament on the following courts

A3W5KQA3C - Mario Cup

M1C2YQM1W - Luigi Cup

LA98JRLAR - Waluigi Cup

UOUFMPUOM - Wario Cup

J6M9PQJ6U - Nintendo Power Cup

MM55MQMMJ - Donkey Kong Cup

OF9XFQOFR - Peach Cup

N24K8QN2P - Bowser Cup

48HWOR482 - Mariotennis.com Cup

ARM6JQARU - Blockbuster Cup

J6M9PQJ6U - Nintendo Power Cup

Castle Court
Two unlock the second Mario Bros. court you will need to beat the star tournament with all 16 characters. This court is in the court yard of Peach's Castle.

Better Characters
Beat the 1P mode singles tournament with any character. When you go to the Player Select Screen, you will notice that that player's token will be shiny. That means that you can press R and give them a handicap.

Super serve with Toad
Tap the controller stick and hold A to do a super serve with Toad that is always in! ONLY IN DOUBLES TOURNMENT!!!!

Beat Piranha Challenge
When playing the Piranha Challenge, choose a character that is fast and/or tall. (i.e. Luigi, Waluigi...) When the game starts, immediately run up to the net and stay there. If your character is tall enough, he/she will be able to return any ball, even ones with an orange trail. But what about the opponent, you say? Well, here's the trick. Whenever a ball is shot, return it toward the piranha plant that shot it, so if the left one shoots a ball, return it to the left. If you keep this up, the opponent will become confused and won't be able to return a single shot you make. But careful: you've got to be quick!
(NOTE: If you're having trouble reaching the ball, look at the piranha plants. Just before they shoot, they cock their heads in the direction they're going to shoot. This should give you a good head start.)

Power Players
Once you have beaten the game game with any character, press and hold the R button. Once you've done that, your player's serve is the fastest. Using the smash shot serve makes the ball go over 100mph.

Really easy points with Daisy
It is simple all you have to do is go up really close to the net do a drop shot and your opponent will move up and smack it right back at you the you use the double A shot and aim it away from the opponent. It will never go out and the trick even works on intense computer players if you do it right