If you beat Bowser 3, after the credits Mario will say "Thank you very much for playing my game!" and "THE END?" appears. If you have all shine sprites, "THE END" appears and Mario says "Hey, you're pretty good! See ya next time!"
Go to the main menu (with the three buttons) and type in "cheat".
Unlock everything: Obvious.
Toggle: Load a save, go on the star map. You can click on Shines and Star Coins to get them (if you want to click Shines, you have to switch to Star Coins and back - odd bug).
To first Bowser Trap:
Go to the staircase (with the door to the basement) and triple jump on the right wall. Go right until you fall down right over the trapdoor.
To Door to Upper Staircase:
Go on the upper ledge in the Main Room and jump against the right wall. Go right.
To Second Bowser Battle:
Go to the slide, Double Jump before it on it, and while still in air, hold Up and Down at the same time.
To Final Bowser Battle:
You can use this shortcut if you have 1 Shine or more.
Reach the top of the main room. You can use Rocket Fludd, or double jump off the middle door and use Hover Fludd - Just reach the top of the room.
Secret Passageways in Castle
A couple of ways can lead you everywhere in the castle. Here are the entrances:
Shifting Sand Land Room: just jump in the hole in the left wall.
Shortcut: Go on the upper ledge in the Main Room and jump against the right wall. Go right. Do a Triple Jump, the last jump right before the end of the platform. Enter the star door.
Basement, Room with the Boo Painting: Activate the switch to raise the water and find the hole near the Boo painting to jump up.
Staircase to upper floor: Triple Jump on the stairs and hit the hole.
Ways get unlocked after beating Bowser. After Bowser 1, you can go to the Secret Course "Thwomp's Fortress". After Bowser 2, you can find the "Power-Up Room", where you can obtain invincibility, Turbo and Rocket Fludd. Also, you can go from there to the Secret Course "The Magma Maze" and everywhere in the castle - use the other paths or use the doors.
Secret of the Mushroom
Jump on the upper middle door in the main room and jump in the (hardly visible) Mushroom Painting - just press Up.
In this course, you can get many 1-UPs - also collect all coins!
Jolly Roger Bay
Go to the elevator room, jump on the left elevator and jump in the left wall as soon it reaches the highest point.
In the basement, activate the water switch and swim up through the hole. Take the Warp pipe on the right.
Secret of the Sky (Rocket):
Left of the Hazy Maze Cave, you can see a sling star in the ceiling, jump it it, shoot up, shoot from the next sling star and you are there.
Frosty FLUDD (Turbo):
If you enter the basement, go right. You can see the outlines of a door there. Enter it to go to the course.
Secret Course. Go to the upper floor and ground pound under the cloud painting.
The Magma Maze
Enter Shifting Sand Land room and take the secret tunnel left. Enter the Bob-omb Battlefield room secret area and jump up. There should be a giant Mario stained-glass window. Now go to the upper-left path, which is cave-like.
Edge of Mushroom Kingdom - FINAL COURSE
Go to the Castle Grounds. Use the cannon (you unlock it with 63 shines) to launch yourself on the roof of the castle. Go to the top and use the slingstar. With the Wing Cap, enter the cannon again and shoot yourself in the preset direction. You reach a ledge. Go right to enter the Course.
Star Coin Prizes
16: Shine Sprite
32: Luigi (enter the door to switch beetween him and Mario)