To do this assignment, think of Boo Radley's life. He has been completely isolated from any socializing, he has a restrictive and stern father and brother that keep him in the house and under their close watch. He doesn't have friends, kindness, or entertainment of any kind. It's a sad, lonely life. In the diary entry, you could discuss how lonely he is, and how he is afraid of leaving the house because it has been so long since he has.
Then, imagine how it would be for him to look out his window, and see Scout, Jem and occasionally Dill, playing happily so close by. He could probably hear their conversations, understand their games, and be highly entertained by their presence. In the diary entry, discuss how fun it is to watch them play. Discuss how you love hearing their imaginative games. And, then, include a couple specific events. One event you could include is when Scout is riding in the tire, and rolls right into his yard, crashing into his house and falling out, dazed. In the novel, Scout hears someone laughing after she falls out--that was probably Boo, who had been watching the event. So in the diary entry, describe that event, and how seeing her fall out and sprawl on the ground, dazed, was so funny that you laughed out loud.
I would end the diary entry by saying that you felt like Scout and Jem were the only friends that you had, and that you wanted to show them that you cared somehow. End it with Boo scheming to somehow give them a gift (which will end up in the tree, eventually). If you yourself put yourself in his shoes, and try to imagine what it would be like, then it will make the assignment a lot easier. I hope that those thoughts helped; good luck!
This exercise sounds like a lot of fun and like something that might encourage you to revisit key points in Harper Lee's novel To Kill a Mockingbird for details that you can then build on. For example, rather than guessing at or completely inventing Boo's thoughts and feelings about the games that Scout, Jem, and Dill play involving Boo Radley (e.g. their dares to enter the yard and even to touch the house or their impromptu street play on their half-invented history of the Radleys), you might want to focus on details such as the laughter that the children think that they hear coming from the house or the loving details that Boo must have put into carving the figures of Scout and Jem.
Of course, quite a bit of invention on your part is necessary for this assignment to work. You'll probably want to invent the precise dates (using the text's cues for guidelines, e.g. the games of the three children take place in the summer) as well as any number of other details that work with the details already given in the novel.