Maybe someone has already suggested something like this, but here's an idea that seems good to me: have a mailbox on our home streets where people can leave things that ONLY the home street's owner can retrieve. People need not become contacts with the recipient in order to leave items such as notes or resources (unlike regular mailboxes).
This way, if someone wants to leave something nice for another player, they can do so, without other folks coming along after them and taking it. When traveling the resource routes, I've often wanted to leave a "thank you" item for those who have set them up, or leave a note saying that I like a street's decoration.
Just a thought... your mileage may vary.