I would suggest that the plans that George and Lennie have are to find something permanent and lasting in a world that is mutable and temporal. The dream that George and Lennie have of owning a small farm in which George could be his own boss and Lennie could tend the rabbits becomes their driving force. In a condition where the "bindle stiff" migrant worker moves from place to place, collecting their "bit" and then going to the next setting, George and Lennie wish to do something else. Their dream is one that features roots. While their state of being is a rootless existence because they go to where the work is, their dreams reflect a hopeful opposite of such a condition.
The material reality in which they experience so much brutality and hardship does not limit their ability to envision a world that is fundamentally different and better from where they are. Their dream is a reflection of this condition. It is one in which roots, optimism, and autonomy are evident, precisely constructed because these elements are not present in the life they are living. Essentially, I would describe George's and Lennie's dream in Of Mice and Men as an example of what life can and should be as opposed to how life is.