The simplest thing you can say to someone is ‘Hi’. It may appear and feel insignificant, but its a memorable and easy way to show those you care about that you see them. It costs you nothing, and means a lot to those you greet.
Just as easy is giving compliments. Tell someone you like their outfit, or think they look good today, or that they’re funny and kind. Nothing is more profound than a simple compliment; people remember how you make them feel. Take advantage of this, and not selfishly, but to make others feel better. It costs you nothing! You’re going to speak anyway, and probably will say something unimportant or forgettable; so, for a change, make it something nice that’ll stick around.