Throughout his play A Doll's House, Henrik Ibsen uses several symbols to portray the theme of true identity vs. appearance. The symbols serve to show that the face Nora presents to the world is not really her true identity.
One symbol is the macaroons in the first scene. The fact that Nora is disobeying her husband by eating the forbidden macaroons shows that while Nora appears to her husband to be a model obedient wife, in reality she is acting in accordance to her own heart and mind. Nora's identity tells her that she is an adult, should be respected, and should not be forbidden to eat treats. Likewise, Nora's personal identity also tells her to sacrifice herself in order to rescue her husband by committing forgery if needed.
A second symbol is Nora's fancy dress. Torvald suggests that she go dressed as a "Neapolitan fisher-girl"(Act II) to the fancy dress ball. The costume symbolizes the time spent in Italy together. Nora played the part of a dutiful wife, pretending that the trip to Italy was for her, but all the while she was concealing the real reason for the trip. The fancy dress symbolizes the portion of Nora's identity that can deceive when she believes it is necessary and will be more beneficial than not doing so.
The third symbol is the Tarantella dance. The Tarantella is a Southern Italian folk dance that represents a victim being bitten by a wolf spider. For Nora, the fast paced, wild movements of the dance represent her being a victim of society. The dance is also another symbol of the time Nora and Torvald spent in Italy and the reasons why. In all, the Tarantella dance symbolizes Nora's appearance as a traditional wife, but further symbolizes the struggles she feels in finding her true self, due to society.
I don't have a copy of the play in front of me, but you'll be able to locate the costume symbolism.
In act two, you can consider the dress Nora wears for the dance of the tarantella as a costume on a couple of levels. It is intended as a costume for the Tarantella itself--a frenetic Italian folk dance that was believed to offset the effects of a poisonous tarantula bite. (The taratella dance, as a symbol itself, is central to the play. As her world threatens to be poisoned by Krogstad revealing her secret and illegal loan, and her life spins out of control around her, Nora is spinning out of control literally in this frenetic Tarantella dance.
The dance also functions as a sort of climactic dress-up scene for Nora, where she will be on display as Torvald's doll wife, dancing for him and their guests. In the folk version of the dance, dancers who have been poisoned by a taratula bite would keep dancing for hours or even days to offset the poison, and stay alive. In Nora's version of the dance, she is also dancing for her life, but the dance is symbolic here. It's a symbol of her final, frenetic, and exhausting effort to keep up appearances and fulfill her role as Torvald's pretty, performing, doll wife.
By sharp contrast, in act three, Nora turns her back on her life as Torvald's pretty pet, and the shift is reflected directly and purosefully through Nora's costume change at the end, just before she leaves. When Torvald reads Krogstad's letter regarding Nora's illegal loan, Nora's literal change into a simple house dress, marks her change in perspective in this scene as well. At the end of the play, when she chooses to abandon her doll life and shows no concern for keeping up appearances, she changes into this simple house dress to begin her journey to find her true self.