To get the Hylian Shield for free, go to the Kakariko Graveyard at night, and pull back the grave with flowers. Drop down, and collect your prize.

The Treasure Chest game in Hyrule Castle can be easily won. The only downside is that you have to wait until later on in the game, as the Lens of Truth is required to win it this easily.

After you have achieved the Lens of Truth, head to the building where this game is held (it's only open at night during childhood) with the Lens of Truth. Play the game and use the Lens of Truth to look inside the treasure chests. This way, you can see which treasure chests contain the keys needed to progress and which contain the participation prize Rupees inside. You will be able to reach the Piece of Heart at the end with no problems whatsoever...unless your magic runs out.

Can only do before you get the Zora's shapphire!

First go to Hyrule castle and sneak past th guards. Then sneak past the guards in side the grounds until you get to a place called the courtyard. The courtyard will have a young princess Zelda and 3 windows, Zelda will be standing at the forward window so go right and look through the window...
Mario!

You might have to move your self and press up on the C button to see better.
The Paintings are set out like this:



[Yoshi] [Peach] [Mario] [Bowser] [Luigi]