[Please provide the exact location of the extract in the novel to easier help you.]
Since I was not sure of the exact location of the extract you refer to, and because you are comprising details to better visualize George and Lennie in the novel, and their environment (as one would with an introductory exercise), I have concentrated on the beginning of the story.
In Chapter One of John Steinbeck's novel, Of Mice and Men, Lennie is described in the way he drinks with the following simile:
...[he] drank with long gulps, snorting into the water like a horse.
Another simile provides information as to the men's environment, before they go to the ranch:
A water snake slipped along on the pool, its help head up like a little periscope.
Lennie is described again in the following simile:
Slowly, like a terrier who doesn't want to bring a ball to its master, Lennie approached, drew back, and approached again.
A metaphor describing the scenery is:
The flame of the sunset lifted from the mountains and dusk came into the valley...
Each simile or metaphor provides an image to better visualize the characters and/or their environment as presented in the novel. These lend themselves to characterizations of George and Lennie, and the story's setting.