Enter the room where you can change into different characters and jump into the glass window of Peach next to Yoshi's door (on the right). You will enter a Castle Secret with three Stars. To get all three stars: make it to the bottom of the slide to get the first, make it to the end of the slide in under twenty seconds for the second, and get every coin in the level (including blue ones) for the third.
Enter the castle, run up the stairs, and turn right to find a brown door with a star symbol and the number "1" on it. Enter the room if you have one star and look to your right to see a big window made of a stained glass Princess Peach. Jump into the window to find a track course. Do the track course to receive a star. Also, before starting the course, look to the right to see a gray block on the floor with a blue coin in it. Do a Ground Pound and press A + R, then enter the course and begin. There will be blue and gold coins on the track. Collect as many coins as possible to earn an extra life towards the middle of the course. When you cross the finish line, you will find a yellow box floating in the air with an exclamation point on it containing a star. Before collecting the star, walk straight towards the gray wall and jump into the wall to make an extra life appear. Jump into the wall immediately or you will not be able to receive the extra life. Afterward, get the star.