Dubrovnik : 1 1/4 mile walk or bus ride from Gruz harbour gets you to the old walled town. In dubrovnik there is loads to see. the wall walk costs approx £5.50 per person.
Corfu : again a walk or bus ride of 1mile will get you to the centre off corfu town. Nice place to wander ~ church of St Spyridon, museum of asian art and the old and new forts are all easily accessible.
Cephalonia: nice place to have a beach day ~ taxi to Lassi beach approx Euro5
Katakolon: This is the one place I'd recommend the ships tour to the olympic site & museum. Not a great deal in Katakolon itself.
Athens : Either ships tour of Taxi to the train station and a train to athens then a wander round the acropolis & the other various sites.
Go to your local library and have a look at the cruise port guides.
Greek tourist office website: www.gnto.gr full details of museum, ruins etc & entry prices
Dubrovnik tourist office website: www.croatia.h.