I've found myself thinking about this from time to time, how little things we can do for others can make their day.
My dad is a Vietnam vet. He is retired and spends his time reading or doing yard work. My mom works 5 hours north, so they don't exactly live together, leaving him alone much of the time. He likes the time alone to read (he is the epitome of a bibliophile), but no one likes to be alone all the time. This past Memorial day, his neighbor's 13 year old son stopped by with a plate of food- a few hot dogs, a hamburger, some other classic summer foods, and thanked him for his service. It was a very small gesture, but it made my dad's day. I think he talked about it for a week.
I was trying to think of the last time a perfect stranger (or close to it, such as a neighbor) randomly helped me with something. I can't come up with anything right now. ---EDITED--- we left a bag in a Starbucks a few months ago- and they made every effort to find us, then held it for us when we returned to see if it was there (we realized we left it about ten minutes after we'd departed and were in a good panic about the possibility of losing the bag and it's contents). I can, however, think of the last time I helped others (tourists often need hints here in the city, and I'm happy to help).
When was the last time someone did something for you that made your day?
When was the last time you did something for someone else that might have made their day?