Making someone smile spreads happiness all around.
This is all about doing little things for people just for the fun and pleasure of it. Not big stuff, just little gestures, like a warm sincere smile as you pass a stranger in the passage, drawing a smiley face on a Post-it note and leaving it on a colleagues desk, sharing your chocolate bar with the receptionist, making eye contact and giving a thumbs up to a colleague who has just had a rough time in a meeting. Any little gesture you can make that acknowledges the person in a caring way brings happiness, not only to them, but to you as well.
This can have a spin-off of creating a happy, more relaxed atmosphere, and it is surprising how much more creative, innovative and engaged people are when they are happy.
This website has a number of easy ideas to get you going. They are so easy you could start in the next few minutes.