There's a particularly wonderful simile in the opening pages when Steinbeck is describing the flat, featureless landscape against which the subsequent action will take place:
On the sand banks, the rabbits sat as quiet as little gray, sculptured stones.
This is a particularly effective simile as it conveys the sense of quiet and stillness of this part of the world. Straight away, we're given the impression that this is a place where not much happens. The stage is being set for George and Lennie's sudden arrival in this oasis of calm, which thanks to them won't be calm for much longer.
Talking of Lennie, here's another animal simile which perfectly encapsulates his character:
[B]ut he's sure a hell of a good worker. Strong as a bull.
The first part of this statement is indeed true. But it's the bit about his being strong as a bull that really captures the imagination. For Lennie is indeed strong; it's one of his main characteristics. In fact, he doesn't know his own strength, which gets him into a lot of trouble.