You know the happy mask sales man in Zelda Majora's mask. If you look at his bag closely you will see a picture of Mario. And also you will see a picture of the mirror shield.
You know the enterance to Lake Hylia. Well make sure you have 5 or more bombs. You know the fences? well put a bomb in the middle around there and a secret hole will appear. If it won't work on the first time keep trying. Also inside that hole a Deku scrub sells you a heart piece for 10 ruppes.
You know where you meet Zelda for the first time. Well if you do look at one of the walls and you will see pictures of mario and other mario characters. Also there is a window that you could shoot with a sling shot and if you shoot it some person will throw a bomb at you.