For this toddler themed party think "Old MacDonald meets Mother Goose". Young children love farm animals and the sounds that they make. Make your entrance into a barn door by covering it with red paper, draw black vertical lines to designate the boards, and paint a white board on the diagonal. Get bales of hay for the party room. If concerned about the mess...you know I'm going to say it, don't you? Put down tarps!
For farm themed games for the tots,use plastic eggs and scatter them around the room, making sure they are all very visible. Let the children gather them and place them into a basket in the center of the room. Give them cotton batting in squares or tubs of cotton balls to play with. Stick a picture of a cow made of felt on the wall, give the wee ones pink felt udders to stick on the cow, some will, some wont - or stick the tail on the pig. Provide a variety of stuffed animals, make sure there are more than enough for each child – they have not yet developed the ability to share yet.
If you happen to know a farmer, maybe he or she would be willing to bring at least one of their farm animals over - though some children at this may be afraid. And that's okay. They could also bring a tractor over instead.
And you definitely have to sing "Old MacDonald Had a Farm" - unless it is like at my house. EVen from a very young age, my son would cover his ears and shake his head no at me if I attempted to ever break into song...