I'm strongly considering bringing along snack size candies when I shop so I can give them a tiny bit of cheer for their day. I figure even if they don't like the 'grab bag' candy flavor I give them, maybe they'll put it in their pocket and give it to a friend when they're ranting on their break about the frustrations of their day but they'll have a nice story about the customer who gave me a candy to cheer me up.
I know I loved it when IT clients I helped would give me a candy or even a flower picked from their after lunch walk as a thank you for fixing their computer problem. I never expected it from anyone, didn't give special treatment/access in return for it...it was just nice to have any gesture of recognition for the work I did. (Goddess knows most bosses only seem to know how to give negative feedback!) It just always brightened my day to know that someone cared.