Super Mario 64 DS

Avoid Damage
You can avoid damage from hitting the floor from a high elevation in two ways: Ground Pound when you're about to hit bottom or fall in Slide Attack position.
(Thanks to Antisocial)

Deceive Klepto the Condor
You can get Klepto the condor off your back permanently if you make it grab your cap while in the guise of another character. Just make sure you won't be needing the cap for the rest of the level.
(Thanks to Antisocial)

Defeat Mr I and Mr Blizzard
Mr. I (the giant eyeball) and Mr. Blizzard (the snowball-toting snowmen) are the only two enemies you can defeat by running circles around them while they have you in their sights.
(Thanks to Antisocial)

Get More Coins from Whomps
You can score an extra 5 coins from a Whomp if you hop on its back 5 times before pounding it into nonexistence.
(Thanks to Antisocial)

Get More Blue Coins
Stomping a giant Goomba in Course 13 yields a Yellow Coin. Ground Pounding a giant Goomba in Course 13 yields a Blue Coin.
(Thanks to Antisocial)

Grated Walls
More often than not, any, and I mean ANY, grated walls you meet can be passed through with a dematerialized Luigi. A great way to cheat yourself to some tough Stars.
(Thanks to Antisocial)

Helicopter Jump
You can only perform the helicopter jump by stomping Spindrifts and Fly Guys, unless you're Luigi, who can perform it at will. Hold B while spinning like a helicopter to slow your descent even more.
(Thanks to Antisocial)

Make your Character Fall Asleep
Your character will fall asleep after they look in all directions ten times.
(Thanks to Antisocial)

Restore Health
When damaged, find a body of water and swim under. After, simply swim back up to the surface. Your health should replenish.
(Thanks to l_is_real@hotmail.com)

Rewards Multiplied
If you swallow something as Yoshi, you'll yield whatever amount of money that enemy or crate normally rewards multiplied by how many enemies you hit consecutively with that same egg.
Example: Swallow a Goomba = 1 coin. Hit three other Goombas with that same egg = 6 coins.
(Thanks to Antisocial)

Tox Box
You can topple a Tox Box in Course 8 as Wario by attacking its insides when it cups you.
(Thanks to Antisocial)

Unlock ? Blocks
Collect at least 14 Stars and enter the Castle Lobby. There you will find a ray of light shining onto the floor. Stand there and using first-person view, look into the light! You will be transported to Tower of the ? Switch where Mario will automatically start with the Wing Cap. Fly down to the center tower, where there is a large ? Red Switch. Jump on it, and all the ? Red Blocks will now be solid in each level.
(Thanks to l_is_real@hotmail.com)

Unlock Cannon
To access the cannon outside the castle near the lake, get 150 stars. There are three 1-ups, a cap and Luigi's rabbit.
(Thanks to Fere_Eversor@hotmail.com)

Unlock Keys
Luigi Key: Beat King Boo in Luigi's Painting on the 3rd Floor of Big Boo's Haunt.
Mario Key: Beat Goomboss who you will find behind the 8-Star Door in the Rec-Room.
Wario Key: Jump into Wario's Painting with Luigi in the Mirror Room and defeat the Giant Ice Bully.
White Room Key: Catch all the Shining Rabbits.
(Thanks to l_is_real@hotmail.com)

Unlock Mini-Games
You must find the rabbit with the specified character.
Bingo Ball mini-game: On the right side of the castle when facing it with Wario. (Near the trees)
Boom Box mini-game: Near the main bridge with Yoshi
Bounce and trounce mini-game: Near the entrance to Lethal Lava Land with Mario.
Concentration mini-game: Near the entrance to Whomp's Fortress with Wario.
Connect the characters mini-game: Near the entrance to Lethal Lava Land with Mario.
Giant Snowball Slalom mini-game: In the alcove across from Rainbow Ride with Wario.
Hide and Boo Seek mini-game: Outside near the main bridge with Yoshi.
Intense Concentration mini-game: Near the entrance to Tick-Tock Clock with Wario.
Lakitu Launch mini-game: Near the entrance to Tiny-Huge Island with Wario.
Lucky Stars mini-game: Near entrance to Bob-omb Battlefield with Luigi.
Mario Slot mini-game: In the secret room with the Mario painting with Luigi
Memory Master mini-game: Near entrance to Wet Dry World with Luigi.
Mix-a-Mug mini-game: Near entrance to Lethal Lava Land with Yoshi.
Mushroom Roulette mini-game: In area near the columns that drain the moat with Luigi.
Pair-a-Gone mini-game: In the hedge maze with Luigi.
Picture Poker mini-game: In the courtyard over the entrance with Luigi. Psyche Out mini-game: In a corridor near Lethal Lava Land with Wario.
Puzzle Panel mini-game: Outside of the main bridge with Yoshi.
Puzzle Panic mini-game: In the moat after it is drained with Yoshi.
Shell Smash mini-game: In the moat after it is drained with Mario
Shuffle Shell mini-game: Near the entrance to Snowman's Land with Mario.
Slots Shot mini-game: In the courtyard behind the fountain with Wario.
Sort or 'Splode mini-game: Near the ledge over the waterfall with Mario.
Super Mario Slot mini-game: On top of the Castle with Luigi. Must have 150 stars.
Tox Box Shuffle mini-game: Outside in a flower patch near the wooden bridge with Yoshi.
Trampoline Terror mini-game: Near the entrance to Wet Dry World with Mario.
Trampoline Time mini-game: Near the entrance to Jolly Roger Bay with Mario.
Which Wiggler mini-game: Outside in the hedge maze with Yoshi.
(Thanks to l_is_real@hotmail.com)