I've cruised with both celebrity and royal carribean in the Med. I used to take the ship tours at first because we were afraid of the ship departing without us. Met some cruisers on the ship who advised that we book private tours for less cost. So I always recommend private tours.

I would recommend a private tour in Istanbul is possible. Visit Hagia Sophia, Topkapi Palace, Blue Mosque and the Grand Bazaar. Skip Spice Market unless you want to buy some spice. I was not too impressed there though as the Grand Bazaar was so much bigger.

Goog luck there.

Frankly, if I were taking a cruise in Europe, that would mean that my interest was not at all in going to Paris (or Berlin or Madrid, etc.). In my opinion, the whole point of taking a cruise is to visit the port cities.