Scratch

If you've played Super Mario 64, or the DS version, then you'll know it's a very complex game with plenty of secrets. If you'd like to know a few, this is a good place to stop by. I know quite a few, but for now, I'll share two I found out of today.

First Secret (DS version, but might apply to 64, too)

You know that hall where you'll find a Boo that scurries away to outside of Peach's castle? Well, follow him, and go through the door he goes through. You'll enter a room with a bunch of ghosts. Now, the ghosts have coins inside them, but eight of them have red coins inside them. As Mario, you can punch the ghosts with the red coins inside, and once you get them all, you'll get a Secret Castle Star.

Second Secret (DS version only)

This is related to the first secret. Out of all the Boos in that little courtyard, there will be one that is bigger than the others, and he doesn't have a coin inside him, but a little miniature tower/house. Punch him, and the house will fall to the ground. Once you approach it, you'll be taken to a secret world. Enter the haunted mansion, and if you go on the right path (I don't remember the exact directions), you'll see a portrait with Luigi in it. Enter it, and there will be a room with a mirror. Go right in front of it, and the Mario in the mirror will transform into Luigi, who is trapped in there by Big Boo. If you succeed in battling Big Boo, you will get the key to Luigi's door in Peach's castle, allowing you to play as Luigi.

But of course, I don't know all the secrets to SM64 and SM64 DS, so share some of yours!