One way to approach this assignment is to do some research on the Great Depression and the migrant workers. However, it is worthy of note to know that the group about whom John Steinbeck wrote his Of Mice and Men is the itinerant white male workers who were displaced so greatly during the Depression.
You may also wish to learn about Woody Guthrie, the itinerant folk singer and voice of the disenfranchised. As Steinbeck himself wrote of Guthrie,
... Harsh voiced and nasal, his guitar hanging like a tire iron on a rusty rim, there is nothing sweet about Woody, and there is nothing sweet about the songs he sings. But there is something more important for those who still listen. There is the will of a people to endure and fight against oppression. I think we call this the American spirit.
The men of Steinbeck's novella exhibit this will to endure. So, in a diary entry, you can write about the men's having ridden on the trains and buses to find jobs, just as Woody Guthrie did. These "bindle stiffs" are mistrustful of others since they all are competing for a limited number of jobs. Fights and such were not uncommon on the boxcars as men traveled across the country. Loneliness is also a companion of these men who work harvesting hay and wheat and other grains.
As you brainstorm for ideas to write in the diary, consider what the men do on a typical day, their fatigue from working, their anxiety that a job is nearly finished and they must search for another, and their mistrust of others. Think about the interplay of the characters in the bunkhouse in Steinbeck's narrative and all the emotions exhibited in it: antagonism, despair, boredom, loneliness. What little hope they have is in the dream of owning something of their own and in having some fun in town. And, yet they endure.
(see the links below for more ideas)