Super Mario 64 on N64 - The Stars of Princess's Secret Slide

In the main hall, climb the big staircase facing the entrance and go all the way to the right of the small balcony to take the door (you need at least 1 star). There, jump into the stained-glass window on the left to reach Princess's Secret Slide.

Finish the race in under 21 seconds and a star will appear the moment you cross the line.

Finish the race in as much time as you like and go smash the yellow block at the bottom.