Early on in my evening shift at the library, I helped an older gentleman with his requests. We didn't actually have the book he desired, but I found something else for him and we had a nice chat and a few laughs in the process.
Later, just before the library closed for the night, the same gentleman returned to the reference desk and presented me with a gift card for a local coffee shop: a thank you gift for my help. Frankly, I was stunned. I was just doing my job.
Thank you, sir, for your kindness and generosity.