Here are a few suggestions:
Chapter 1, when Ralph is being described:
You could see now that he might make a boxer, as far as width and heaviness of shoulders went, but there was a madness about his mouth and eyes that proclaimed no devil.
This foreshadows that Ralph is basically good at heart. There is no sign of the devil about him. From what we know happens in the rest of the novel, this will turn out to be a prophetic description. I don’t want to give away the story, if you have not finished it – so you will have to trust me on this one. This is an important quote.
In chapter 2, the boys are collect leaves, twigs and sticks to build a rescue fire on the mountain. After they collect piles of stuff, they soon realize they don’t know how to start the fire. A fire is symbolic of civilization and the boys will become less and less civilized as the novel progresses. They are going to do things that are shameful and that require “confession”:
“Ralph and Jack looked at each other while society paused about them. The shameful knowledge grew in them and they did not know how to begin confession.”
In chapter 3, there is a paragraph that describes Jack as he begins to hunt for food – if you did not know it was describing a human, it would sound as if it were an animal. You could choose any of the sentences from this paragraph, but one in particular:
“Jack himself shrank at this cry with a hiss of indrawn breath, and for a minute became less a hunter than a furtive thing, ape-like among the tangle of trees.”
The imagery is pretty powerful and we see Jack as a hunter/animal, which will also prove to be true as the novel progresses.
Read about it here on eNotes.