At Ancona you can walk directly into the town but it is of limited interest. At Corfu there are rows of taxis to take you into Corfu Town for a small price and exploring there is fabulous. At Piraeus it is best to take an organised trip and if in Athens, I would suggest the Acropolis is a must if you haven't been there before. Sorry haven't been to other ports.
Not true about Pireaeus. Just get off the ship and walk to the Metro 9about 15 mins). Get a return (16 euros) to MONISTRI. If you ask anyone on the metro they will tell you where to get off. It takes about 20 mins. You then have a lovely walk slightly uup hill to the Acropolis. Entry to all the sites is 12 euros. Try and get there as early as possible as all the caoches arrive about 11 and its a bit chaotic. As the Acropolis is now covered in scaffolding we found it a bit dissapointing. When you walk back down visit all the little shops in the surrounding area. We caught the 2pm metro back and had plenty of time. Can't help you with the other ports but as a general rule I would always say do your own thing it is much cheaper. All these taxi drivers have been taking cruisers for years - they know where you want to go and the fastest way to get there. Pompeii if you go is easily reached by taxi. The metro also runs to Rome. It is very easy - just outside the port and very fast. You can be at the Colloseum before the cruise tour is half way there. Enjo.
Just returned from 2 weeks cruising around greece, cyprus, turkey, croatia etc
Dubrovnik - fab scenic place, our cruiseline provided shuttle buses into the old town, it is too far to walk, but taxis available.
Corfu - easy to get a taxi from the port to the town or to a beach
Piraeus - if your cruiseline puts on a bus which takes you into athens then lets you wander around on your own for a few hours before taking you back to ship then that would be a good compromise. We did it alone and agree that the subway is easy efficient and clean taking you straight into the centre of the city, depending on where your ship docks it is anything up to 2km walk to the subway station in busy streets no signs etc. it took us 45mins, so on the return journey we got a cab to take us back from pireus station to the ship 10 euros i think it was much easier. The acropolis was mobbed hundreds of tours pantheon shrouded in scaffolding, but great views over the city and there are other buildings for photos. Good shopping in athens even an m & s! Dont underestimate the heat.