Compassion. Sometimes that's all kindness is. Maybe it's taking that little extra time explaining features to a new customer. Maybe it's holding a hand and being quiet while she cries. Maybe it's walking slowly behind a senior citizen not in any rush to get to where you're going.
Thinking of others first. Seeing that another person has value. Letting another person, be exactly who they were meant to be, with your support, patience or encouragement.
Compassion. Another way to show kindness without spending a dime. Put yourself in someone else's shoes and act accordingly.