Seating

If its made clear that I shouldn't wear my dressiest clothes, I would be fine sitting on hay bales. However, chairs are pretty cheap to rent. I rented chairs for my backyard event for $3 a piece. Good luck!

Keep in mind that chairs have backs for support, they're not quite as low to the ground as hay bales, so for older guests or guests who may have issues with back/legs/etc a chair would be better for their comfort/pain management. For example, my dad has a device implanted in his back to help with back pain, and he almost always needs to lean against a chair back for support when sitting, so he would not be able to sit on a hay bale. My grandmother has weak knees and a hay bale would be too low to the ground for her to get up and down easily and it would cause her a lot of knee pain even after just two hours.