Let us start off with fire and its significance in the novel. The way in which fire is presented makes it become a symbol of the cruel and evil power of the Nazis. Consider the way in which Madame Schachter has her vision of the fire and the destruction and death it forebodes. Eliezer also sees Nazi soldiers burning Jewish babies. Of course, most significantly, the fire becomes a symbol of what happens to the Jews and how so many of them meet their deaths. However, fire also becomes symbolic of the way that Elie lost his faith. Consider the following line:
Never shall I forget those flames that consumed my faith forvever... Never shall I forget those moments that murdered my God and my soul and turned my dreams to ashes.
Thus we can see fire also becomes an important symbol of the way in which Elie's experiences killed his faith.
Secondly, as the title signifies, night itself is an incredibly important symbol. Night is used as a symbol without the presence of God, and you might want to analyse the way in which the description of the most intense suffering in this novel normally occurs at night to reinforce the way in which God is absent from this hell-like world with which we are presented.
Lastly, you might want to consider Jewish tradition, and how its importance is symbolised in characters such as Cholmo and his storytelling. Note the way that Elie struggles to maintain his links with Jewish tradition even though he does not believe in God any more. For him, and for many like him, their Jewish tradition is symbolic of unity and togetherness with his fellow prisoners and Jews, and he is faced with a massive conflict when he debates the question of fasting on Yom Kippur or not. Such traditions form a valuable, life-giving link for Jews like Elie to remind them they are part of a people who are bigger than what is being done to them.