I absolutely love this thread! I have two, and soon will have three, units, so you can multiply these all by that! They are all very cheap.
My ideas:
In our area many bars/restaurants/clubs/events give out decals (stickers) for laptops and waterbottles. I always take a few and keep them in a bowl in the unit. I have a sign that says 'take some Raleigh swag home!' and have had guests comment on that and take one or two with them. It's a nice way to remember their trip. Cost - FREE
I collect a copy of our local free weekly every week and put it in the unit. Cost - FREE
I have snacks in each unit, in pretty clear canisters or jars on the open kitchen shelving. These look pretty and keep the kitchen from looking too empty. There are a few granola bars in a jar, another jar with a few packs of nabs (what Southerners call those packaged Nabisco crackers with peanut butter and cheese on them) and another one with dum-dum suckers. There are also a few packs of ramen noodles in the cabinet, plus oatmeal, hot coco, coffee, tea, etc. I also put individual milk and almond milk containers in the fridge. If a guest will be with me for a while I will put a few fresh eggs (we have chickens) in a bowl in the fridge, too. Those are almost free. Cost- about $20 month, at most, as most folks don't take them, but they do leave reviews commenting on 'all the snacks' I leave out.
I have told family and friends that I will happily be the repository for all their unwanted paperback and hardback books. I have a lot of readers in my circle, so they give me books pretty regularly. I have bookshelves in each unit and a sign that says 'if you start a book on your visit, please feel free to take it home with you!' and many have commented on that. Cost - FREE
I have a small basket of toiletries in each unit as well. I do buy some items, but I have also told family and friends I love sample-size and unused travel sized items that they don't need. My dentist also gives me toothpastes and brushes with her office card attached. I have sent two guests to her for emergency dental calls, so she thinks of this as good advertising. Cost - FREE