Oh! Oh! You're visiting me! Yay! What? Mario told you I'd help? Oh! Of course! Yay! I'll tell you about Secret Levels!
Level A - In Level 1-2, collect the Propeller Mushroom. Then, in front of the Flagpole, fly up and over the flag. Then, walk into the door on the fortress and you will see a pit. Walk into it and you will land on another flag. You will enter Level A.
Yosh! Yoshi yosh yosh! Yoshi yosh yoshi yosh yoshi yoshing yosh yoshi yoshi yoshiyo! (Hi! I like you! I will help you by telling you about secret characters!)
Luma (all colors) = Enter a level. Any level. Then, get a Mini Mushroom. Then, get a Propeller Mushroom. Spin into the air and fly over a pit. Fall into it. Your Luma will act as if you're still there. Luma is also invincible.
Yoshi (all colors) = Enter World 2-2. Get a Yoshi. Then, eat a Mini Mushroom with Yoshi. Yoshi will shrink and you will disappear. Yoshi will then grow back and you will be Yoshi. Yoshi is invincible.
Wahahahahahahaha! I'm-a Wario! Heh... WHAT?!?!?!?! You want to know how to find MY treasures?! Well! Gr...FINE. Here you go!
Golden Mushroom = Go to World 5-4. Go to the tower of sand, then collect the propeller mushroom. Fly to the top and fly again. Land on the clouds. In front of you is a normal mushroom. Fly up and land on the mushroom. It will turn golden! Collect it and you will be invincible for the rest of the level.