I'm not a cabin steward, but I'm in another profession that gets "appreciation gifts" from people -- I'm a teacher.
First of all, I'd like to say that I actually prefer NOT to get Christmas gifts, end-of-year gifts, Teacher Appreciation Day gifts from students. It always makes me uncomfortable, since there are inevitably some students whose parents don't follow these traditions and I feel badly for those kids if I *do* make a big deal out of the gifts, and I feel badly for the kids who bought them if I *don't* make a big deal of it. So, my preferred gift is a letter thanking me for whatever it is I have done for their child that year (assuming they believe it!). It was particularly nice the year that all the parents got together, wrote a glowing letter, and sent it to my superintendent, asking it be put in my permanent file. :-)
That said, however, I do have some preferences about gifts, if I must recieve them. The sad fact is that I have enough knicknacks and Christmas ornaments to decorate five houses. A very, very small percentage of the housewares I've been given fit in with my decor. I'm not a "novelty" kind of person, so those aprons, vests and earrings with little books and pencils hanging on them are in a box in my closet -- I feel badly about giving them away, but I can't see myself wearing them.
I guess what I am saying is, "personal" gifts usually don't work well in a professional environment. While my students' parents know me as a teacher, they don't really know my personal likes and dislikes as well as they think they do. I've generally found that the gifts that I have been able to *use* the most have been pretty generic -- a girt certificate to the local mall, bookstore gift certificates, etc. I know the parents often feel they are "copping out" by not making it more personal, but in fact, it is a better match for me.
Because of this, I'd say that cash is the best form of appreciation for a cabin steward. You don't know how much opportunity he has for shopping on land, you don't know if he has anyone to call on the mainland. Of course, if you've struck up a conversation with him and know, without a doubt, that he ADORES the Red Sox, a cap might be nice, but I doubt you'd have one in your luggage! ;-)
I know that feels less "personal", but I would be willing to bet it would be more appreciated.