Farms require lots of different workers to get the jobs done. Offer a brief explanation of some of the jobs -- milking the cows, feeding the livestock, herding the cattle, etc. Offer some farmer and cowboy outfits for students to use for dress up. Then ask them to imagine that the room is their farm. Decide, as a group, where the barn is, where the farm house is, where the sheep or cattle are, etc. Then give the children a chance to improvise doing the activities required for any of the jobs you have described. To keep the room safe, each child should mime riding horses, feeding the chickens, carrying buckets of milk, etc.
What is it like to be a farm animal? Create a list of farm animals with the children and write each animal on a small slip of paper, fold it, and place it in a hat. One at a time, have the children come to the front of the room and select a paper. Whisper in his or her ear which thing they have chosen to act out. Have him act out the animal for the others to guess. Encourage the children to create the sounds that it makes and to move in the way that it does. Have a time limit in place, in case there are no correct guesses. In this case, return the paper to the hat, so that someone else might try.
Farm animal puppets can be purchased or made out of simple supplies like socks or Styrofoam balls and popsicle sticks. Once you've got your animal puppets, have the children create a farm puppet show. Encourage them to create voices for the puppet they are manipulating. How does a goose sound? A cow? The old, crabby farmer? You might want to let the children play with their puppets in small groups and then have each group present a puppet show to the others based upon their play. Observe their attempts to create conflict and stories with a beginning, middle and end. Take note of these efforts, telling them that they are already learning the skills they'll need later in school as they begin to read books and write papers.
Read the children a story about a farm life and farm animals. Discuss with them the characters in the book. How were they portrayed? Ask for volunteers from the group to play each of the characters in the story and, with some simple dress up costumes, have them re-enact the events of the story that you read to them. Have them be sure to include the beginning, middle and end, explaining to them that every story has each of these things. You might organize your simple dramatic performance into "Acts" -- Act One as the beginning, Act Two as the middle, and Act Three as the story's end.