Something I've been making an effort to do more is to try to have a softer look on my face when I'm in public. I tend to be pretty intense looking, especially when I'm thinking hard, which I usually am when I'm walking or standing in line.
I've noticed that consciously thinking about something that makes me very happy, or something I find funny, will make that happy emotion show on my face. When I project happy into the world, I get a lot more happy back - people (I'm talking about adults here) seem drawn to strangers in a good mood. When I'm consciously trying to be happy, people are much more likely to smile and say hello.
And maybe it's confirmation bias, but when I'm trying to project happiness, and when I try to really smile at people and say good morning and mean it, I notice a lot of times that people seem to walk away with a little more pep. Almost like "wow, some stranger just genuinely wished me a good morning."
This all could be cause I live in Santa Monica and people out here tend to be happier than they did in NYC.