Big Boo In the Blackness
In SM64, after beating Bowser for the second time, go through the door (with the keyhole) leading to the spiral staircase. Turn around and go back out the door and head for the room with the entrance to level 3, Cool Cool Mountain. After going into the room, take a right and go down the ramp. If done correctly, you will hear Big Boo's shrill laugh. Go out of the room and head to the long hallway where Big Boo usually is. He won't be there. Now open the door to the courtyard and in the blackness will be Boo.
***If you find anything else that this code does, please contact the Nintendo 64 Codes and Cheats!

Hat Trick
In all of the levels in which you can lose your hat, lose it. Then, go through the warp a few times. When you are done, go find your hat. It should be a stack of hats! Take one, and Mario will put it on. Take another, and Mario will take off his hat and swing it like a weapon. Be careful, because you lose a lot of strength when hit like this. Also, you can't carry your hat for any more than that one level without doing the trick again.
Also, in Snowman's Land, if you lose you hat, get a star, then turn the N64 off, a snowman will be wearing your hat next time you enter the level!

Secret Room
After you have gotten all 120 stars, go outside the castle. Hop in the cannon and shoot yourself onto the roof of the castle. Get the wing cap and fly back down to the cannon. Get back in and aim toward the small cloud above the middle tower of the castle. Glide Mario to the middle ledge of this tower. Get some speed then run into the wall. You will fall through if you hit the right wall (the one on the right when you stand by the cannon) you will fall through down to the bottom. To get out, go into the castle and walk through the wall next to the door.

Whomp Secrets
In Whomp's Fortress, there are two cool little things you can do with the Whomps. Instead of killing them, jump on their backs, and they will give you a coin (up to five). Also, when they are about to fall, you can jump up through their faces (it takes a bit of practice, though).

Get to the Roof Without 120 Stars
When you first start the game, go up to the path, make a right and follow the path across the bridge and head for the last tree. Start doing a triple jump, making sure your second just is right at the corner before the hills goes up vertically. The third jump should get you to the top of the hill, and you'll start sliding down. Quickly jump while you're sliding, and with luck you'll grab the top of the castle wall. Yoshi isn't up there yet, but you can get three extra lives and the flying cap to play around with.

Butterfly Fun
In the courtyard in front of the castle, you can gain 1-Ups by butt-stomping the butterflies which are flying around.
Near the beginning of Tiny-Huge Island, if you butt-stomp the butterflies there they will turn into huge cannonballs and explode.

Crazy Lives
Using the Japanese version of SM64, collect 1000 coins during the Bowser levels (get coins that are left from his breath) and each time you die you will gain a life! (Same thing goes for when you gain a life -- you will lose one.)

Water Levels
You can change the water levels in Wet-Dry World by entering the painting at the top or at the bottom respectively.

Giant Penguin
After you have won all 120 stars, go into the Cool, Cool Mountain course. Choose the level of racing the penguin. Once you enter the chimney, you will see a huge penguin.

Tower Secret
After you have defeated the Whomp King, enter Whomp's Fortress again. There should be a tower in the place of the Whomp. Go to the back side of the tower (with the shadows) and punch the wall. The wall will breaks apart, revealing a one-up.

Fall back into the Cannon
On the first level, activate the cannons, and go to the one that's just a hole in the ground near the base of the mountain, and jump in. If you aim at the right spot on the little cloud above the mountain, you'll hit your head on the "ceiling" of the sky and fall back down into the same cannon.

Meet Yoshi
After you win all 120 stars, go outside the castle. The cannon should now be open. Blast to the roof of the castle and you will find Yoshi. If you talk to him, you will receive 100 lives.

Thanks to Vicky Martin for sending in these codes!!
Get down to half life or less and go into the vanish cap place. Long jump so you land near the edge at the very bottom. You will lose all your life but you will slide off the edge. Mario will then immediatly drown.
To get on the castle and get the Wingcap
Fly down to the cannon. Shoot yourself onto the higher ledge. Now move to the side facing the waterfall and run up it to get to the very top of the castle.

Thanks to Sphere for sending in this code!
If you lose your hat and then go get a wing cap it will never run out!
*note you can use this by loosing your cap in gameshark.