I've written longer letters, but on my holidays this year I sent two postcards to a lady I met at a nursing home. I wanted to let her know that when my husband and I visit with our dog, that we aren't just volunteering in a generic way, but that she is special and I think of her as a friend. So I crammed a bunch of writing onto a postcard while I was away, and ended it with 'thinking of you'. There were two, and because her eyesight is not great, the staff read them out loud to her during the mail hand-out.
Now when we visit, it's mutual hugs all around; it was one way to say she is a friend and because they were read out loud, everyone else knows she is special too.