Argostoli, Kefalonia, Greece - Make the trip to a beach. Very nice! But go early to get a spot.
Venice, Italy - Get ready to walk and deal with crowds.
Corfu, Greece - Lovely beach on other side of town. Take public transit outside of Pier, very reasonable.
Piraeus (Athens), Greece - Amazing. Don't get suckered in by taxi drivers. Take red hop on and off bus. Very reasonable kids are free and very thorough.
Mykonos, Greece - Not much to see. Long lines for taxis. Beach rocky.
Kotor, Montenegro - If you walk up mountain, wear appropriate footwear. Can be dangerous, not suitable for young children.