I to have agree with the phone card thing. That's what I'll be bringing on my cruise along with a cute little Christmas card to tuck them in. Our gratuities are almost always pre-paid, so I always found it a nice gesture to give these "extra" gifts to someone onboard that really went out of the way or did a really superior job or made my trip even more special. I try and do something like this year around on all my cruises. It all started with me on DCL , folks that sail her even started a club with a membership pin and compliment letter to give the CM's onboard that they want to recognize. Some of the things I have brought for them are small trinkets from Disneyworld (that actually say Disneyworld) like keychains, small excutive writting pens, ornaments...etc. But, keep in mind even a small compliment letter also works well if you really need to do something in a pinch. The last person I gave one too (again on DCL) the next time I saw him said they actually gave him a small raise due to all the compliment letters he got, so that must work too. I don't think RCCL folks will appreciate my Disney trinkets, so I am sticking to the phone card thing...it's a pretty univerally used gift.
Enjoy your cruise..I know I will!