You can use the Hop on Hop off bus in Athens. No problem with that. We didn't use any ships excursions on our cruise last November. We did everything private.
We didn't do any excursions in a couple of places but as far as Sorrento be careful what you book. If you are going to be staying in that area fine, but if you are going over to Naples the highway is cut off due to work being done on a tunnel/underpass. It makes the trip a lot longer. We ended up going back to Sorrento by train from Naples. (They call it a train - we call it a subway car.) It was OK. Took about an hour. That was the only time I wasn't happy with our driver. He took us everywhere we wanted to go but didn't tell us he was only returning us as far as the train station. Nevertheless, it was wonderful to see Naples again.
Celebrity Constellation - March 17 followed by Celebrity Reflection - March 22
44 cruises - too many to list however cruiselines are in no particular order:
and some unknown tub
Yes, I'm a Royal Chump and proud of it.