To match your invites, create a barn door on a wall - interior or exterior. Set up a stool or haystack (even better) in front to have the kids have their pictures taken. Pass out straw hats as part of the party favors to make the pictures extra cute! Maybe even put a few farm stuffed animals around to finish off the scene. Source
This is a pic from the party I threw my nephew. I blew up white balloons and colored "cow spots" on with a permanent marker. Super simple (and easy to pack in the suitcase!).
I thought that feed bags would be really cute but I didn't have a sewing machine at the time... I took pieces of burlap and stapled them all the way around with a regular office stapler - no lie! - and turned them inside out and put bandannas in them and wrote "FEED" on them. No one ever knew my "no sew" secret (until now) lol...