I have been thinking about this a lot. We have hosted almost 100 guest stays now and a few of them really stand out as being awesome guests.
In addition to leaving our place clean, following rules and communicating well, the "super guests" do things like wash their towels and fold them with a little note that says "we did these in the laundry so you don't have to." We always leave a treat (food or wine) for our guests and some have left us something in return or a nice note thanking us for the stay. A few have even taken their trash with them rather than leaving it for us to pick up.
Beyond that, great guests don't lock themselves out because they didn't follow basic instructions (especially more than once) or over-communicate about things that aren't issues ("I don't know what all your light switches turn on. I went around and counted and I can't figure out what one does" ). Great guests have read and understood the house rules and are respectful and courteous.
We talk a lot about things guests do that are difficult to manage on this board. What have guests done that you really have appreciated or have made them stand out as fantastic guests?