Basically letters. Simple notes that you can stick on the gates, the door or the nearby tree for your villagers, mates you live with, friends, neighbors and enemies. Easy to make, easy to use, or perhaps reuse it if it is a parchment. Maybe it's only me, but I'm constantly catching myself thinking that I need to say something to my friends and neighbors who are not in the game at the moment or to my neighbors that I don't know but want to. Building a plate or carving a runestone is the ugly and expensive way to do so.
It would be cool if we could build mailboxes and send and recieve those letters via them. Let's pretend that pigeons, falcons, owls and other feathered creatures transport them around. Or better make them to actually carry the letters and one can shoot the bird to intercept the message and take over a strategic secret or a romantic confession.
Also it would be even more cool if we could stitch sheets together and make books and journals, memorize an event (screenshot), then engrave it on the parchment and then gift it to someone or keep under the lock.