For a 300-word paper on the symbolism in Henrik Ibsen's "A Doll's House", you may want to focus on one specific set of symbols that bring out the central themes of the play. These symbols can be verbal or physical.
One interesting set of verbal symbols are the pet names and terms that Torvald uses for Nora. He often describes her as small animal, a lark or a squirrel. Although these terms appear affectionate on the surface, it is worth exploring how they trivialize her and treat her as less than fully human and less than an equally partner in the marriage.
Another interesting symbol is that of the forbidden macaroons. You can think of these in terms of symbolizing a system of arbitrary rules of power or also of the notion of circumscribing and disciplining desire in a Foucaltian sense.