I lived in Paris for two years. I arrived there when I was 17 and I found that city beautiful and magical but there was something sad about the people who lived there everyday. They looked sad, even depressed (some of them probably were)...I myself suffered from severe depression when I lived there.
In the underground, people would avoid looking at each other, as if they were scared of what would happen if they made eye contact with someone. I know it is going to sound silly and it's not really one specific act of kindness, but I had this "game" to make my day nicer and brighter : I would sit (or stand) in the train and try to make eye contact with someone, anyone. And then I would just smile at that person. It's nothing really, but the feeling I had when the person smiled back or even was into starting a conversation with me, was amazing. Even if I had had a terrible day at work, this made me forget all about it and, deep down, I knew, it brightened that stranger's day too.
In a big city like that, with such big crowds, people are lonelier than they look. I felt lonely and depressed and I knew I wasn't the only one. I just wanted to see more people smiling where no one was.