HI Judy
If I know that the couple staying are on their honeymoon or it is a special wedding anniversary I always leave two small bottles of our local sparkling bubbly (not champagne) with appropriate glasses and they really appreciate the gesture. I also always leave a chocolate on each pillow - doesnt cost much and is appreciated. I do think that bath robes are essential. If you have time it is always nice to leave some home baking in containers. It goes without saying to have a guest directory with pertinent information such as restaurants, markets etc.
I am well aware that a lot of hosts on Airbnb do not include breakfast and there has been many a debate over this. Personally, I think that hosts should provide a simple continental breakfast so that the guests can start their day right. It doesnt cost much and it saves the guests hunting around for a breakfast place when they could be out and about doing and seeing things. My husband and I spent 7 weeks overseas lately and stayed mainly on Airbnb and only a couple of places provided breakfast which I feel is quite mean. A lot of our guests recommend us to their friends because of what we provide. After all, surely Airbnb does mean 'bed and breakfast'!
Hope this info is of some help.