This is a great question! It made me sit down and think through so many kindnesses! I think the one that stood out above the others was one done a few years ago by my youngest stepson.
We had a snow/ice storm in north Texas. Most neighborhood roads were closed as they could only really maintain the larger roads. I was "working from home". Now, I have a favorite drink - in fact, this canned soft drink is about all I drink. Well, in the middle of this storm, I ran out. Of course, the kids and their friends had helped me find "the end" as they'd not been expecting to be home and celebrated the holiday at our house. We all kind of laughed and tried to think up some alternates I might want to drink til the weather cleared.
Sometime later that day, I saw this child walking up the driveway with a case of soft drinks. He'd bundled up and walked all the way to the nearest opened store - which was a few miles away.
Not only had he done this for me - but he'd used his own money to buy them. Wow. My heart still smiles just thinking about it.