This may not sound like much, compared to some of these, but it really made my day.
It was back towards the beginning of my first year of uni - I was in the depths of studying for my Christmas exams, plus writing a 3000 word essay that I was hating with a passion. After about four hours of straight studying, I needed a break. I wandered down to the library to get out a few more books, and stopped off at the uni shop on the way back. I picked up some milk, and, while putting the change into my purse, dropped the books. I almost burst into tears (it was that sort of day).
The man behind the counter came out, and helped me pick them up.
"You look like you need something happy today," he said, smiling. Then he reached over to the display on the counter, picked a strawberry lollipop and handed it to me. "Go on, I'll take care of it."
Again it may not seem like much, but it made me very happy.