A number of friends are crew - I know they appreciate small snacks as gifts from well-meaning passengers (provided no expectations are attached). I've worked in hospitality for over a decade - staff providing a service are only human and it's nice to feel like customers are appreciative (even if you are just doing your job).

On most LH flights (flying in Club or WT+) I bring a £2-3 bag of individually wrapped chocolates for the crew - it's always been well received and I do it for no reason other than an effort to display a bit of thoughtfulness. Even after years of flying - travel can still be exciting (sometimes) and enjoyment shared is multiplied.

With the obvious exception of comments about gift-cards and cheap attempts at getting upgraded - some posts above about well-intentioned treats are miserly. "You can easily judge the character of a man by how he treats those who can do nothing for him". Technically a good crew shouldn't be able to do anything more for you anyway (you should be well fed, watered and attended to - regardless) - they can't affect the flying time, weather or pre/post-flight experience. Heaven forbid that it's possible to be nice for no reason!

A final anecdote: On a recent Club flight to Singapore I didn't get my first meal choice on the basis of bringing treats for the crew - a female crew member came back after taking my order and advised she was 1 short on the beef, but as "I seemed like a nice person would I mind swapping for the chicken". Having had the beef 3 weeks before I wasn't bothered, so it goes both ways.