Remember that a symbol must be an actual "thing" and then also suggest something else. I think the most obvious symbol in the novel is the key that Oskar finds in his father's things. Oskar becomes obsessed with finding the lock that his key unlocks and he searchs all the buroughs of New York in his effort to solve this mystery. But more importantly, solving this mystery of the key, helps him work out his issues about his father's death in the Twin Towers during the attacks on 9-11. He needs to find a "key" to unlock his feelings of love, loss, and guilt. This mission becomes the key to his emotional healing. Through his adventures with this key, he meets new people who have more difficult lives than he does. He meets people in whom he can confide or learn about life. He comes to better understand his paternal grandmother (and grandfather) who actually share some of the same trauma that he does. We learn later in the novel that Oskar's mother has "arranged" his mystery with the key and that the key doesn't open anything of any true significance. While these details may be a bit of a letdown to the audience, it is actually part of how the key and the lock are symbolic. The key to Oskar's finding peace had nothing to do with a key -- it was the experience of the search and the passage of time. The lock wasn't meaningful for the same reason.