We have a septic tank so since I first started hosting (ten years ago!) I have always had signs in every bathroom asking people not to flush anything but toilet paper. A few years ago I bought some nice hand-painted signs to put on the shared bathroom upstairs and the extra toilet downstairs, as I noticed guests sometimes got confused between the bathroom and bedroom doors.
Then a year or so later for the same reason I stuck on each guest room door a postcard with a strong simple and distinctive image - flowers, a ship, some Moroccan tiles - with colours that match the room decor (and the key fob is also colour co-ordinated). So even tired, jetlagged and disorientated guests can easily identify which room is theirs.
I leave a "Welcome!" leaflet on every bed thanking them for visiting, with the address, phone numbers, wifi password, house rules, a list of local eateries (and phone numbers, and times of opening), and local attractions, what to do in a fire or medical emergency, a reminder about check out time, breakfast times...anything that might be useful for them to know - the sort of things that I'd like to know if I was a visitor.