Cash can be a crowd pleaser, but not always appropriate.
"happy anniversary honey, here's 50 bucks."
Okay, the relationship between you & your competitor and you & your wife might be a little different, but getting the cash into the right hands might be tricky.
I'm a small shop so I can't advise from that perspective, but if it were me I'd be concermed about two other issues. If I did stop by, would I be bothering someone who is also too busy to hear a thank you. And what if it is not well known throughout the company that your name is being given out? Not saying either scenario is likely.
All in all I think it is worth the risk to go with the Starbucks idea, but I'm still curious to see what others think.
