We did this for my daughter's 3rd birthday.  It is a great outside party. As each child arrived, they were given a bandana to tie around their neck. We had a rocking horse, hayride (on a riding lawnmower with trailer attached filled with hay), sandbox filled with small farm animal toys that the children could dig for, bales of hay placed around that the kids loved jumping on/over. We cooked out hot dogs, etc. then rented 2 ponies for pony rides. The cake was a sheet cake with crushed oreos for dirt and mini veggies made out of marzipan (think edible playdo) and a scarecrow (Popsicle sticks and felt) held a sign "Happy Birthday Caroline".