Not that much to do in Piraeus if you didn't plan to go into down town Athens.
We took a taxi and went to the Knossos Palace. Very interesting even without a guide. 3 and 4 storey buildings thousands of years ago!!
Took an independent tour to the Sea of Galilee and it was very interesting. Might be better to see this area and the area of Jerusalem and surroundings other than as day ports from a cruise ship.
For all the controversy over three major religions having their holiest places in Jerusalem, it is so commercial that it has lost it's authenticity. Maybe the problem is that it cannot be done one day from a cruise ship and needs to be a destination to allow the time to get into the history and savor the experiences.
Valetta and all of Malta is the most amazing city/country in the Med. It is like sailing back into the time of the crusades. You have to work at finding a modern part of the city that argues with the complete sense of history that you feel when you are there. We have been to Malta twice and loved it both times. First time took a hop on hop off to the Blue Grotto. Amazing sights and history everywhere you look. One of our top 5 favorite cities in the world.
Have been there before and there isn't much to do right in Naples. We took a walk through the upper town and financial/shopping district to see the Christmas decorations. Lovely day and good weather.