Attain 120 Stars and then...
After getting 120 stars, go in the cannon outside on the castle grounds. Launch up onto the castle, and you will find Yoshi. Talk to him, and you will receive 100 extra lives. Your Triple Jump will also look different. From on top of the castle, you can use the Wing Cap.

The only other change that has been discovered after 120 stars, is the penguin in the Penguin Race. He has put on a little weight since last time you saw him.

Ending Camera
After beating Bowser, plug a controller into port 2. Use the analog control stick to manuever the camera during the ending sequence.

Hat Weapon
This trick has been found to be accessible in two different levels so far. It allows Mario to make multiples of his hat, so he can use one as a weapon.

In Snowman's Land, have your hat blown off by the giant snowman when crossing the bridge. Don't pick it up...just remember where it blew off. Next, from the area of the cannon, walk right, past the small snowman, and into the corner of the level. You should see a couple of trees. Stand by the one nearest the corner, and you should teleport. Teleport back and forth a few times, and then go see your hat. Pick up your hat, and there should still be one on the ground. Pick that one up too, and Mario will hold it in his hand. Push the punch button, and he will swing his hat. Unfortunately, you lose the hat weapon when you exit the level. If you save it without your hat on, when you enter the level again, the snowman in front of the cannon will be wearing it.

In Shifting Sand Land, let the vulture take you hat. Kick him so he drops it, but don't pick it up. Stand by the tree in the water, and you should teleport. Teleport back and forth a few times, and then go see your hat. Pick up your hat, and there should still be one on the ground. Pick that one up too, and Mario will hold it in his hand. Push the punch button, and he will swing his hat. Unfortunately, you lost the hat weapon when you exit the level.