One of the biggest hints at Nora's cleverness is the fact that she had both the idea and the gumption to actually forge a signature on a legal document. A lot of women in her same situation would not so easily have been able to figure out a solution. Nora was in quite a desperate situation because at the exact same time that the doctors told her that Torvald's health was in serious danger, her father was dying as well. Nora was very prudent to feel that she ought not to worry her dying father with her husband's ailment. However, in her era, women were not allowed to take out loans without the signature of a man. Therefore, it was very cunning of Nora to figure out a way to both rescue her husband and spare her dying father from further suffering by forging her father's name.
While I would hesitate to call Nora manipulative in the sense of intentionally, underhandedly trying to control or manipulate situations for her own advantage, she certainly does know how to steer a situation in the right direction. Her ability to maneuver situations is another hint that Nora is clever. One instance of her ability to control a situation can be seen in the very first act. After Krogstad blackmails her and Torvald returns home saying that he has figured out that Krogstad has been asking her to put in a good word for him, Nora redirects the uncomfortable moment by asking Torvald to help her choose her dress for the fancy dress ball. In fact, she intentionally makes herself look silly and as if she can't possibly get on without him, as she says, "I can't get along a bit without your help" (I). The effect is that she smooths things over between Torvald and herself so that she is more at liberty in the future to ask Torvald not to fire Krogstad.
Nora also attempts to maneuver a situation in order to smooth it out later on when she distracts Torvald with practicing the Tarantella in the second act. Asking Torvald to devout himself to helping her practice her dance serves to postpone Torvald's opportunity to read Krogstad's letter. This distraction buys her more time. We especially see her intentions in her lines, "You must not think of anything but me, either to-day or to-morrow; you mustn't open a single letter--not even open the letter-box--" (II).
Nora knows that Torvald has control of the mail. She also knows that the letter from Krogstad about the loan and crime she has committed is in the mailbox. Nora illustrates how clever she can be when she pretends that she needs Torvald's help with the tarantella, the dance she will perform. Nora dances violently to keep Torvald's attention and keep him away from the mailbox.