Finger food: sandwiches, fruits, veggies, pop, water, etc. Info flyers or pamphlets outlining rates, amenities and employee/owner qualifications. If you'll have a barn vet or farrier make them available for people to meet. I don't like being restricted to a specific vet or farrier but want to know who will be called in an emergency if my personal choices arent available for whatever reason, who's coming to do routine shots on a shared call etc. If you have any barn rules make sure they're outlined upfront.
Have boarding contracts available for viewing. Maybe raffle off a free or discounted month if you can afford. Or a year contract with get your 12th month free/discounted. Referral discounts- ie: you refer someone who stays for x months (6?) get x amount off your next bill, or a gas card/tack store voucher? Retention of good, honest people is important and people also want to and should be rewarded for their loyalty.
If you have amenities that other local farms don't, highlight them. Do you offer hauling? Where and what's your rate? Are you catering to a specific group? What kind of notice do I get if board is going to be raised? Horse care expenses are rising constantly right now. I understand that, but will be upset if I am not given a chance to plan for the increase in my bill too. Oh also what kind of grain and hay is available? Put out info on the hay nutrient tests and the grain percentages. Will you provide a maximum amount of feed/flakes of hay before you charge extra? How many times a day will you feed? Make all info easy, upfront and leave little to the imagination on what is realistically being offered. You don't have to entertain, just inform.