Also did 12 days on Millie and sailing out of Venice was a highlight! Be sure to be on deck early to grab a good spot becuase it is special.
Aside from that cruise I have done others in the Med but dont have much to add to the first reply you got.
However if I can help - I gladly will.
Croatia - Dubrovnik. The ships shuttle takes you to the Old City - which is full of character but small. However we were happy doing the walls (hope its cooler for you) and milling around the old city. Apparently the ships excursion alongthe coast is very beautiful.
In Athens - which we had done before on a land based holiday - we used the metro. A comfortable walk from the Ship. And we went back to the Plaka and shopped. In the Plaka there is also a little tourist bus/train that takes you to a few sites. Nothing major but a welcome respite for tired feet.
Of course you have to do the Acropolis and depending on how much of a history buff you are - u may want a guide.
Mykonos - very walkable. Tho I am sure we only saw a v small portion of it. However time was limited on our cruise.
Rhodes - LOVED the old town. And sounds like you will be one of the scooter brigade - hire one of the yellow jobbies and tour the island. I cannot coordinate scooters and the such like - so I stuck to the old city - happily.
Venice - you dont need any tours. Take the water busses - walk - have a guide book - and a great time.
Any questions? If I can I will help.