I borrowed an idea from a neighborhood I once visited that used a front-yard chalkboard for rotating positive messages. I wrote a massive, colorful "Have a Beautiful Week!" in sidewalk chalk right where people walk their dogs.
I received two incredibly sweet texts and a handwritten card tucked into my own mailbox just saying thank you.
Leave a quick, positive message or a thank-you note in their mailbox just to tell them you appreciate having them as a neighbor.