I've given away upgrades several times,
including one on DEN-HNL where I traded seats twice and ended up on E+ rather than F on the UA flight. The purser actually came back and chided me for that move as it confused them during the service.
And there was that time
I used my AmEx $200 airline credit to buy all the drinks any passenger ordered on a flight from Anchorage to Honolulu. The look on the FA's face when I tried to explain that I wanted to buy 30+ drinks was a classic.
When I see tourists who appear lost in NYC I'll offer help. Sometimes it is just pointing them in the correct direction. Other times I'll walk with them to their destination and chat along the way. Great stories nearly every time.
n.b. The link above is to my blog or to one which I am a regular contributor. FT rules require that I disclose that in the post.