Play as Luigi, Wario, Waluigi and a lot of other characters.
Luigi:
Go to the castle courtyard and jump on every boo (including the big boo) without touching the ground. When done, you will here a sound. Next, go to tiny-huge island and change sizes 64 times. On the 64th time, Luigi will jump out instead of Mario.
Wario:
Do the same as above, but as Luigi. After doing that, you'll here a sound. Go outside, and Wario will be standing there. He'll then feed Luigi to his pet bob-omb and then you can play as Wario.
Waluigi:
As Mario, battle the 1st Bowser. The second you start, you have to grab onto his tail within 1.987465342 seconds. Once done, spin Bowser around 128,784,902,167,256,792,389,267,569,832 times and throw Bowser off the edge. If done curectly, Mario will sat "Hey, Bowser! Get a life!" and Bowser will jump up, unzip his costume and become Waluigi. Then Waluigi will throw Mario off the edge and say "Hey Mario! You suck!"
8-Bit Luigi:
1. Make some green jell-o
2. Cover you N64 so that every last spot is covered with green jell-o
3. Take a giant hammer and smash the jelly into your N64
4. Turn on N64 and Mario will appear covered in green jell-o. he'll then transform into Luigi
Geno (from Mario RPG):
First, put Mario RPG into your N64 and beat it without using any other of your party members' attacks, besides Geno (you must use a game shark to get Geno at the beginning of the game) After that, save the memory onto your N64 memory card and play Mario 64. Geno will say "Hey, thanks Mario for helpiung me kill Bowser!" The screen will go black and you'll here punching noises. MArio will then be gagged with a koopa shell and tied up.
Bowser:
1. Beat the game 64 times without getting hit and under 15 minutes
2. After beatin the 3rd Bowser the 64th time, Bowser will say "Yo, Mario! How do I keep coming alive? This is been the 64th time!" and MArio will say "Okay, how about we switch spots and you can try to win Peach's affection and I can try to kill you to steel Peach!" Now you can play as Bowser
Master Hand:
Buy SSBM on GCN and beat Master Hand 64 times with each Mario character. Save the game onto a memory card and then put the memory card into your N64 controller. Then tape SSBM onto your M64 gane cartridge and play it. Now get all 120 stars, beat Bowser and visit Yoshi. Instead, Master Hand will be there and he'll say "Soon you'll meat a bunch of other Nintendo characters and kill me, so I'll just kill you now." He'll give 99 poison mushrooms and you'll die.
Wamarigemasterbowuigiwa (all characetrs put together):
Earn all of the above characters and beat the game 1234567890 times.
Note: All of the chracters have their own stage. Luigi has No-Mario Land, Wario has Greedy Gardens, Waluigi has Confusion Place, 8-Bit Luigi has Super Mario Bros. World, Geno has Magic Mounain and Bowser has Big, Bad, Angry, Killerish, Destruction Islnd and Master Hand has Final Destination. At Final Destination you can play Super Smash Bros. 3!