Dubrovnik is an amazing city. If you want to walk the walls go as early as possible to beat crowds and in summer it is so hot. Its a city to be wandered round down below as well. Follow signs to cliff top bars. Explore the surrounding shoreline. Eat Pizza!
We docked in Piraeus which is an industrial port. We wandered round but its not that great. If you do experience it keep walking to the far end of the marina and it picks up a bit. There was big queues for the hop on hop off bus to athens as well so a tour may have benefits here.