When you decide to visit Greece there are some things you need to know, not only about the car rental policies, but also about the roads you are going to cross. The highways in Greece are large and generally safe to drive. You can drive to thessaloniki and Peloponnese from Athens easily and without stress by using the highway.
The only thing you have to keep in mind, is to have some change for the tolls. When visiting Thessaloniki with a car rental, you will be able to make some interesting trips. If you want to visit Veroia, you need to have 2.40€ if you want to visit Kozani you will need 7.20€, and if you want to reach mount Olympos you will need to have about 6.80€ depending on the exit that you prefer to take.