One important item or image from the book would be the trout.
And then Miles caught a fish. There it flopped, in the bottom of the boat, its jaw working, its gills fanning rapidly. Winnie drew up her knees and stared at it. It was beautiful, and horrible too, with gleaming, rainbow-colored scales, and an eye like a marble beginning to dim even as she watched it. The hook was caught in its upper lip, and suddenly Winnie wanted to weep. "Put it back, Miles," she said, her voice dry and harsh. "Put it back right away."
The fish is symbolic of Winnie's struggle with life and death. She knows that the Tucks are immortals, because they drank from the spring. She knows that she has a chance to become like them, but she is not sure what her choice will be. By demanding that the fish be thrown back, the reader begins to think that Winnie is going to choose everlasting life over life followed by death.
A second important item to the story is the bottle of immortality water that Jesse gives Winnie. If she drinks it, she'll live forever. If she doesn't, she'll live out life like a normal human. The choice is super tough, but in the end Winnie decides to use the water to save the toad. By pouring the bottle out, the reader is meant to assume that Winnie chose to live a normal life.