As a quick update: my wife and I I handed out $5 Starbucks gift cards to the FAs on my flight (I handed them to one of the ones working my cabin who distributed them to the rest) and I think every one ended up stopping by to thank us personally during the flight. They also gave us a couple of amenity kits (we were in economy) as well as a bottle of champagne and a hand-written thank you note from the purser (alas we had to throw away the champagne at LHR since you have to go through security every time you connect there). But more importantly, they seemed genuinely shocked and delighted with such a small gesture. I think I might make this an annual thing!