[Susie] But he grew tired of hearing me plead. He reached into the pocket of my parka and balled up the hat my mother had made me, smashing it into my mouth. The only sound I made after that was the weak tinkling of bells. (10) Susie's jingly cap is a homemade symbol of her mother's love and care. Harvey perverts it, using it to gag Susie while he rapes her. It becomes a symbol of her loss of breath and voice. When Len Fenerman recovers her hat and shows it to Abigail and Jack, it becomes a symbol that great harm has come to Susie. 

[Buckley to Jack:] "I saved the Monopoly shoe and then it was gone. You took it. You act like she was only yours!" (60) Buckley is about four when Susie dies, and, while he never completely understands what happened to her, he has a fair idea. When it seems clear to the family that she's met a bad end, Jack tries to explain the absence of the beloved sister to Buckley over a game of Monopoly. The Monopoly shoe was always Susie's piece. The removal of her piece from the game is what Jack uses to illustrate to Buckley her removal from the game of life, and he gives it to Buckley as a memento of his sister.

[Jack and Buckly] He smashed that one first. "Daddy?" Buckley said. My brother held the doorknob with his hand. (41) But, when it turns up missing it becomes, for Buckley, a symbol of what Susie's death has taken from him namely, his father's love and attention throughout the years. In the difficult moment, Buckley begs him to let go of the dead and be there for him, the living. The very idea of stopping his continued vigil for Susie is so threatening to Jack that he has a heart attack.

[Susie] In the days that followed I watched my family walk back and forth past that drawing and I became convinced that that thick blue line was a real place-an Inbetween, where heaven's horizon met Earth's. (17) The “Inbetween” is where Susie resides in her Heaven. It is the place between Earth and Heaven where she and her family are stuck. They can’t go forward into the future. It’s representative of Purgatory. 

"We still miss you," he said in December 1977, when the leaves had all fallen and been blown or raked away but even still, with the ground waiting to receive it, there had been no snow. (213)  Finally the Icicle in reality, it is only frozen water that can eventually melt away. To Susie, it’s a softer way to die. For Mr. Harvey, it is death itself. I couldn’t find this in the book but in the beginning of the movie Susie looked up at a harden icicle, it calmed her in a strange way. At the end though, that icicle melted and it’s what made Mr. Harvey slip and die.