This isn't one that I've odne, this is one that someone did for me.
The other day I was waiting in a queue at Boots, quite visibly sick and shaking with this stupid cold I have. I was kind of wobbly on my feet, filling a prescription and grabbing as many Halls Soothers as my hands could carry.
The guy in front of looked at me struggling along and stepped aside and said "you go first, you need to get home and get rest".
I stared at him in stunned disbelief, smiled a lot and thanked him profusely.
Yesterday I found a cashpoint that wasn't working. Instead of letting everyone try it and walk off, I stuck a sign on it that says "broken, other one works, another around the corner".
Not a huge gesture, but I'd be grateful if people would do that to broken cashpoints.