One good example of symbolism is the notebooks that the main characters use to make their plans. Pudge, the Colonel, and Alaska are always writing in their notebooks; because they are living in the moment, and can't always memorize their ideas, the notebooks symbolize both the growth of their minds and the desire to be immortalized. If something is written down, it becomes more solid and more permanent than if it is simply spoken. After the tragedy, Pudge uses Alaska's notebook to create her final prank, ensuring that her memory will live on for a long time.
Another example of this symbolism is the memorization of Famous Last Words by Pudge. Because his focus is on an unformed ultimate goal -- the "Great Perhaps" -- he doesn't think about how people get to their stations in life, only how they left them. Pudge's obsession with Last Words shows that he has less concern for the journey than he has for the destination, even if the destination is death.
"...his last words were 'I go to seek a Great Perhaps.' That's why I'm going. So I don't have to wait until I die to start seeking a Great Perhaps."
(Green, Looking for Alaska, Google Books)
While he claims to not want to wait until death, Pudge shows that he doesn't really understand the idea of the "seeking," focusing instead on the "Great Perhaps" as a destination. It takes the tragedy of Alaska's death to shock him into taking his journey seriously, and allowing the destination to come to him instead of trying to create it from nothing.