Merci d'être sympa
I like it when the buses of the city happily roll around with rears covered with the following message, illustrated with peace flowers: Merci d'être sympa. Thanks for being nice.
An attitude campaign, not a commercial campaign. I think the intent is to introduce this as traffic rule no 1, but it can safely (and somewhat sadly - in an ideal world we shouldn't need reminders to be nice...) be applied to the entire public space. I'm sure you can remember a time when a complete stranger smiled at you or even said something funny to you just passing by and how that just might have made your day.
I can not help but wonder though whether the bus drivers actually get the message...
So tomorrow, please go out and be nice to a stranger (even if it's the bus driver).