I still remember the faces on the ones who notice while we're still there. We don't get to see the reactions on half of them because we just leave it as a regular tip so most of the time we were gone by the time they found it. But when they notice they usually run us down as we leave and say thank you so much.
My favorite was our first year doing it. We had a family member (not me) who thought the waitress was cute so he left his number. We got a text later Christmas Eve night that she was a single mother of 3 who'd lost her job and was working at the restaurant in order to survive. She didn't have money for presents that year so she was going to go to the store after work and get each of the kids a little candy for Christmas. With the tip she was able to go and buy actual presents. Easily my favorite tradition. I love seeing how happy they are. I hope to continue doing this forever.
A close 2nd tradition is driving around looking at Christmas lights on Christmas Eve. We've done this without fail for decades. We would then end the night with a couple Christmas movies, usually Elf and Grinch.