Super Mario 64/The Princess' Secret Slide

Once you get a star you can go to the room upstairs to the right. Here you will find Mario, Luigi, and Wario, but they're behind locked doors and you cannot open them now. To the right is a picture of Princess Toadstool. Jump into it and you will arrive at the secret slide. Slide down to the bottom to receive a star. Do it again under 21 seconds to receive another star! Here's a hint: to save time, you can jump over the railing on the left at a certain point early on and land on the ending portion of the slide below it. Use the map as a guide.

There is a blue coin switch behind where you start.