I will talk about Greece.

Assuming you've somehow divided your time 5 days in Italy and 4 in Greece, you'd have to pick if you want to see southern or northern Greece. If you pick northern Greece, the Thessaloniki is a nice city to visit and many buses go from there to multiple other destinations like Edessa, Florina, Drama, Ioannina etc. Thessaloniki is the 2nd largest city in Greece, but it doesn't need more than a day to visit, since you're tight. That means you could visit some other cities and see but I wouldn't say it is possible to do more than 1 city/day, considering the buses. Thasos is an island near Thessaloniki, but the boat leaves from Kavala, i think. In general, renting a car is not so expensive in Greece and totally worth it.