One fun thing that my cousin did for my Aunt & Uncle's 50th anniversay was to make them a quilt. They sent a white fabric quilt square to all of their friends and family, and asked them to design it however they wanted. Some people did embroidery, some had pictures printed onto the squares, but it turned out looking very nice. They then put together a patchwork quilt, with colored squares (the colors of their bedroom) mixed in with the white ones.
Of course, you need to have the necessary talent to put it together, which I certainly don't. It might also be a little bit big to take on the cruise, but I know they really appreciated the sentiments from people they didn't see very often.