I ordered KFC two weeks ago. The girl taking my order greeted everyone walking in the door with a smile. She kidded and laughed all the while thru a crazy luch hour. After I got my order, I walked away, then turned around and put a $20 in her hand and told her it was because she was just a nice person. It made her day, but it really made mine.
Done similar stuff and you're right, it makes me feel great and the recipient is always surprised and very happy!