We will be traveling on the same June 29th cruise. Likewise, our family of four has booked private tours in all but Mykonos, Florence and Marseille.
I had a friend that just returned from this same cruise and she said Mykonos was fine to do on your own. She said it wasn't worth going to the beach ( the water was freezing - this was the first week in June) and they charge 20 euros for a beach chair.
I think we are going to do Florence by train on our own and I am leaning towards taking the train to Aix-En-Provence as well. I just can't seem to find any private tours that look reasonable in price.