I think magnets are a good idea! Do you ever do embroidery on tea towels? I have a few embroidered towels that MissingWillow and KittyKill have made me and I adore them. You could embroider veggies on them and they would sell like hot cakes, I bet! Do you do any kind of bath and body type crafts? Something like a balm for gardeners would go well with a farm stand.
Good luck! It sounds like it's going to be a fun endeavor.