I think that the most significant symbol in Alexie's novel is in the title: the absolutely true diary. It's a metahistorical symbol, particularly as it plays upon truth. Books, words, and language (the old textbook thrown at Mr. P) have purported to speak truths about Indians, but these truths were written by oppressors. Also, early in the book, Alexie refers to his cartoon as 'tiny lifeboats' in a world of broken dams, etc. Throughout the novel, the imagery of planets and orbiting also works to add perspective on time and place as a significant theme. And the basketball rivalry speaks for itself, particularly as Rowdy and Junior resolve their differences. They quit keeping score. Hope that helps. There are more, I know, but I think those are significant ones.