Princess' Secret Slide: Extra life:
If you get the first 1-Up mushroom, there is a second 1-Up behind the Star Box's platform.

Tick Tock-Clock: Star:
As you enter the room with the Tick-Tock-Clock level, there is a Toad standing against the right brick column. Talk to him as Mario and he will give you a Star.

How to beat the Goomboss:
As Yoshi, run around the arena as fast as possible and try to get behind the Goomboss. Swallow a Goomba that follows him. Wait for it to become an egg and throw it at him. Repeat this two more times.

Defeating King Boo:
Get to King Boo by going through the Luigi picture in the Boo House. You can tell where he is when he disappears in the mirror by looking for his shadow on the floor.

Defeating Lethal Bombs:
To kill the Lethal Bombs, either jump at them or punch/kick them ahead of time.

Psyche Out! mini-game:
The mini game tests your psychic abilities. There is a trick to beating this game. If you look carefully at the waving card, you should see a faint outline of the image that is on the back of the card. If you can not see the image immediately, wait a few seconds and you should get a glimpse. This will happen more frequently on the harder levels.

Character power-ups:

Mario
Feather: Jump three times to make Mario fly.
Flower: Make mario inflate and float slowly; handles easier than the feather.
Note: Mario is just the basic character with no special abilities.
Yoshi
Flower: Makes Yoshi's mouth full of flame and can breath fire. If you ever see fire, such as in the basement, you can eat it and can spit fire once.
Note: Kump and hold it for about a second. Yoshi will with kick his feet (faster than Luigi) and hover for about two seconds.
Luigi
Flower: Makes Luigi invincible to most things. He can go through screens and other objects.
Note: Luigi can run on water for a short time. Also, if you jump and hold it, he kicks his feet fast and falls slower. If you try to do a triple jump with Luigi it can be more difficult -- if you hold it for about one second, he kicks and you will not be able to finish the triple jump.
Wario
Flower: Metal Wario; he is invincible and can stay underwater.
Note: Wario is the slowest character. He also lacks jumping skills, but he is the best at Ground Pounds. He can break black bricks