Keep in mind that, for your servers and stateroom steward, the tips are their income. They send money to their familes in their home countries, so that their familes have a roof over their heads, food, clothing, and education. So money is the best tip.
Phone cards are appreciated on top of that because they allow phone calls to their familes back home -- and they don't take up much room.
Beyond that, I would be careful in being too creative. Your servers and steward have little use for merchandise or merchadise gift cards. They live in small, shared cabins, and have liitle room or need for "stuff." And, when they need something, money (instead of gift cards) gives them the flexibility to determine where and when they want to shop.