Write a diary-style entry on notable themes, details or quotes from pages 115–159

The first thing to do is find a theme, quote, and/or detail that you think is "notable" from those pages. Different editions of the book will have slightly different page numbering, so I doubt that I can nail down an exact page number for you. Additionally, that is a huge page range for you to choose from. The book is loaded with notable events and quotes. McCarthy's narrative doesn't spend much time giving "useless" information. He moves readers from one graphic and jarring event to the next with fair regularity. If I had to pick something, I really like the sequence that begins on page 125 of my version. It is the sequence where the Man has found the flare gun. The Boy is amazed by the find, and he is genuinely curious about shooting it. He wants to know if it could kill somebody, and the Man replies that the gun would most likely light the person on fire. The Boy asks about it as a signal device, and the Man confirms that wasn't his reason for getting it. The Boy correctly interprets it is because there is nobody left to signal. The Boy then makes a wonderful connection. If there isn't anybody to signal, there isn't any harm in shooting the flare off at night. The Boy says it would be a celebration of sorts. A few pages later, they shoot the flare at night. The Boy then makes a comment that the flare signal isn't something that can be seen from far away. That disappoints him, and the boy then questions whether or not God would be able to see it, and the section ends.
What I like so much about this event is how it is a bright spot of sorts in the book. The flare gun is symbolic of hope. There is hope that good guys might see it. There is a wonder about whether or not God could see it. Regardless of either possibility, the gun also concretely brings a moment of joy for the Boy and the Man. They really have nothing to celebrate, yet the Boy still maintains his innocence and hope; therefore, he still holds on to ideas like celebrating something in a loud and bright way. The flare gun does this literally and figuratively. If your diary entry has to be from your perspective about your thoughts on the book, then I would discuss the flare gun's symbolism.
The diary entry could be a bit more fun to write if you are allowed some basic creative writing freedom. Write the journal entry from either the Man's perspective or the Boy's perspective. You can include all of the stuff about the flare gun being symbolic of something, but you can explore a bit of what McCarthy doesn't write about. Readers get very little information about what any character is thinking about unless that character says it. We get information on what is happening, what they see, what they do, etc.; however, what characters think is minimal. A diary entry from their perspective could explore what the Man thinks about the Boy's suggestion of a celebration. You could explore why the Boy wants to shoot gun but decides that he is too scared to do it himself. Write about what was going through the Boy's head while he watched the flare arc over in the night sky.