Give a free gift to the next person--Smile. It is contagious. Not only will you feel good about doing an act of kindness, you will get to see how your smile actually influence the receiver, the person being smiled at.
Chances are that person will remember there are nice people, therefore causing him or her to have a more positive attitude--possibly influencing the next person he or she may see.
Smiling is free. There is no monetary value on smiling, but its meaning is priceless!