Simile: "...to lay, like a blessing, above the contents, a sheet of rustling tissue against the dust."
A simile compares two things that are dissimilar, and does so with the words "like" or "as". Here, the tissue covering protects all of the items from getting dusty. Whenever things are put into storage, especially in old homes or attics, people cover the items with a sheet to keep it as fresh and new as possible. Here, the author is comparing the sheet to a blessing. To the items, the tissue is like a blessing, protecting it from getting dusty and old.
Metaphor: "...she had a blinding glimpse of molten gold: it was spring sunshine on the pale stones of the hall floor."
A metaphor describes something by stating that it is something else. Here, the sunshine is described as molten gold. For Arrietty, especially because she never really saw sunshine before, the light is beautiful and like gold. It's "molten gold", spreading around the floor, dripping and flowing into all the corners. It is shiny and bright and resembles gold being melted and poured all over.
Simile: "Clear and bright like the color of the sky."
This simile is describing "the boy's" eye. Without directly saying so, the author tells us that the boy's eye is blue. It is also clear and bright, so we know it isn't a rainy, cloudy day, meaning gray eyes, but a "clear" and "bright" day, meaning fresh, blue eyes.
