The Royal Family must vacation in the Mushroom Kingdom
When you meet Princess Zelda for the first time in Ocarina of Time, paintings of characters from the Mushroom Kingdom can be seen through some of the windows in the courtyard! Paintings of Mario, Yoshi, Peach, and Bowser can be seen hanging on the wall if you play around with the camera enough.
This easter egg also translated into the version for the 3DS. Instead of individual paintings on the wall, there is now a scene from New Super Mario Bros.
How the easter egg appears in Ocarina of Time 3D