Scout wears a dress to Aunt Alexandra's missionary tea party in chapter 24. She mostly attends the party because Jem and Dill go skinny-dipping and it wouldn't be appropriate for her to be with them while they are undressed. So, in the monologue, Scout could grumble about missing out on swimming in the beginning. Then Scout offers to help Calpurnia and takes in a coffee pitcher to the women. This is when Aunt Alexandra invites her to sit down with them. There are two paragraphs where Scout describes the women's clothing and make-up which would be fun for a monologue. For example, Scout wonders, "why ladies put on their hats to go across the street" (229). She also explains that she controls herself by "tightly gripping the arms of the chair, and waited for someone to speak to me" (229). These phrases could be adapted to show how Scout not only notices a lot from her childlike perspective, but also does her best to blend in.
Next, you could mention the fact that Miss Maudie notices Scout's dress and asks where her pants are. Scout's response is "under my dress" (229). You could include that everyone laughed as she realized what she had just revealed. It's cute, and again, shows Scout's childlike personality. The cuteness can't go on forever, though, because Miss Stephanie baits Scout with a question about what she wants to be when she grows up. Miss Stephanie says she thought Scout would want to be a lawyer since she's already been to court. The ladies laugh.
Scout then witnesses a very prejudiced conversation dominated mostly by Mrs. Merriweather. Depending on the tone for your monologue, you may want to consider how it develops and ends first. Do you want the monologue to be serious or fun-loving? If you choose the latter, then don't go any further into chapter 24 for material. Not only does the conversation turn prejudiced, but Atticus then comes home with news of Tom Robinson's death. Either event could destroy a happy monologue centered around Scout's childlike characteristics.
If you want to dive into the serious topics of the chapter and contrast it with Scout's innocent behavior, that could be a powerful technique. It's as if Scout was dressed for a party, but she went to a mud-slinging fest and comes out feeling dirty. Then, to add insult to injury, Scout learns Tom died, which would make her brightly-colored dress feel very inappropriate for the sadness of the situation.
Therefore, you could focus on the dress as the center of the monologue. Scout first pretends she is a maid by bringing in the coffee pitcher. Then, she's invited to be a lady at the party, but experiences disillusionment when she winds up getting metaphorical dirt on her dress from all the prejudiced talk being thrown around. Finally, her pink dress becomes a poorly chosen one worn to be worn at a funeral, which is how Scout feels when she learns of Tom's death.