It can change your day. It can change a stranger's day. And it can definitely change a friend's day.
A smile.
I think we underestimate the effect that our attitude and actions have on the people around us. When we get angry, the people around us are affected. When we're sad, likewise. But what about when we're happy? Laughing? Smiling? The same thing happens.
Encourage a culture of happiness. Laugh when you can. Smile if you're able. It's really that simple.