This one is easy - give out five genuine, unexpected smiles. Some people will return them, some will wonder what you're up to, and if you're lucky at least one person will pass your smile along to someone else.
A genuine smile comes from deep within you - find that center of gratitude and joy, and just project it into the world. Even on a bad day, a smile can help turn things around.
Look people in the eye, flash a genuine smile, and move on with your day.