Have you looked at Royal Caribbean

They do cruises around Greec and surrounding areas. I know they also do Cruise and Stay as well in alot of places.

Royal Caribbean are fantastic couldn't fault them in any way, service, food, facilities are all excellent and very reasonably priced!

Don't think they do cruise and stay as such, I know they call at Valleta, Malta though.

Costa also will do cruise and stay but you can stay anywhere and sort out the stay package yourself. The days of cheap low cost airlines make it easy to book the flights yourself. Even start at one Port and finish at another no problem. Your travel agent would do this for you if you are not happy booking there yourself. Never travelled with Costa so can't give any views on them.
Doing 12 med out of Venice ending in Barcelona and using Jet2 rather than Celebrity as their flights are not direct and are double the price.

Sue

Thanks for your replies everyone