In constructing a keepsake box for Miranda, the objects that are placed in it should be representative of her characterization in Wonder. It is essential to find objects that can either represent who she is or embody who she once was. As with a keepsake box, what goes in there has to tell a story. The story of Miranda becomes the primary distinction as to what goes in the box.
One item that Miranda might put in a keepsake box would be her acceptance letter to Faulkner High. It was a moment that could only be shared with Via and Ella. It was the instant in which their social lives would change. The three of them were inseparable in middle school. However, the acceptance into high school- in the form of that letter- was where their social lives changed, and where separation began to take place. While Miranda ends up socially reconciling with Via at the end, the initial separation caused as a result of high school is embodied in that letter and so it would make sense for her to place it in the keepsake box.
Another item that Miranda would keep in the box would be a lock of her pre- high school hair. In order to become socially accepted, Miranda "matures" and devises a look designed to appeal to the socially driven world of high school. She changes her hair color into bright pink and begins wearing tube tops, as means to increase her social popularity. Like the acceptance letter, the lock of her hair in the keepsake box is a reminder of what life was once like. It is a reminder of her past, something that carries with it an entirely different narrative than the popularity- driven life she lives now.
I think that Miranda might also have a copy of the film Heathers in her keepsake box. The film is a dark comedy about how people change once they enter high school. The summer before entering into Faulkner High, it becomes clear that Miranda and Ella spend time with one another, apart from Via, as a way to boost their own social credibility. The film takes aim at this common reality in high school and does so with a sense of honesty and black humor that reminds everyone how real and transcendent values eventually supersede the temporal reality of popularity and social acceptance, a lesson validated by Miranda's and Via's reconciliation at the end of the novel.
Finally, the last two items in the keepsake box would have to relate to how Miranda loved August. At one point in time, Miranda was defensively protective of August, even lying to tell others that he was her brother. Miranda and Ella used to play dress up with August when they were younger, so I think clothing that was used to dress up August would be another item in the keepsake box. It tells the story of how caring and compassionate she used to be towards one who did not experience it from many people. Along these lines, the astronaut helmet that Miranda gave August shows a true love that she displayed towards him. The fact that August ends up wearing the helmet often is a testament to the devotion she felt towards him and him reciprocating it. The clothing from dress up and the astronaut helmet should be in the keepsake box because they both independently tell a story where feelings and emotions mattered for Miranda. Sentiment and loyalty were more important than the fleeting reality of social acceptance and popularity. Keeping these items in the box is a reminder of what she used to be, the good and honorable person that was and still exists at her core's essence.