I created a Farm theme for my son's 3rd birthday party. Here's how I did it.
Decorations - I used plain colored plastic table cloths in primary colors and white. On the white one, my older kids colored cow spots with a black marker. On the yellow one, they drew a duck. I bought balloons with farm animals on them from our party supply store, and my finishing touch was a hula hoop with streamers that I hung from the ceiling in the kitchen above where the food was served. I got lots of compliments on that. It took a while to do, but it was so cheap! And you can use whatever color streamers will go with your theme. I stuck with the primary colors.
Food - I tried to come up with creative farm-themed names for most of the food I was serving. Little Red Hen's Chicken BBQ, Tractor Wheels for the sandwich cookies, Haystacks for crisp rice treats, Chicken Feed for party mix, Old McDonald's mac & cheese, etc. To label the food, all I did was print tags from the computer onto brightly colored paper, again sticking with the primary colors.
Cake - I take pride in, and enjoy making my kids' birthday cakes. For this cake I just made a rectangular cake and then used food coloring to make the vanilla icing green. I iced the entire cake in green, which became the farm field. At the back of the cake, I placed a pound cake covered in chocolate icing and licorice strands to make it red. I used 2 graham crackers for the doors and pretzel sticks to make the roof, and it became a barn. Then I washed a couple of my son's plastic farm animals and put them in front of the barn.
Activity - The best part of the party, though, was making scarecrows! Everyone brought their own clothes for their scarecrow, and I supplied the pillowcases to make the heads, plus the straw to stuff them all. There were some very creative scarecrows! One person used just a pillowcase and made it into a cat, and another person turned their pillowcase into an owl. I found reward certificates at the dollar store, and so each scarecrow won an award for "Sportiest" or "Best Animal", etc. I had other activities planned, like playing "Duck, Duck, Goose" and coloring farm pictures from a coloring book I had bought, but everyone (adults and kids alike) had so much fun with the scarecrows, that I didn't even need the other activities.