I write to my villagers quite frequently, especially since I just learned how to send presents, but just because I write often doesn't mean they are exactly well-written, haha.
I talk about something interesting that is happening. Be that the change in weather, something new in town, something I did, other villagers. If it's a gift, how I found it, why I'm sending it, etc!
I really like sending letters, I used to write a lot in my old town. I need to get back to it!