Most car rental agencies in town are on Syngrou Fix avenue, close to the Acropolis Museum. But driving out of Athens can be very stressful if you're not used to it. There may also be traffic jams depending on the time.
Without going to the airport you can find other car rental agencies outside the city center. It all depends on where you want to go next and whether the car rental company is right for you.
If you go to Peloponnese (Nafplio, Delphi, Meteora, Olympia) there are several local car rental agencies and also Hertz and Budget in the Maroussi district, north of Athens. where you will not have to deal with the traffic in city center.
Do a search on Google Maps for “rental car maroussi athens”
They are right next to the E94 or E75 motorways which go towards the Peloponnese.
This saves you from wasting time returning to the airport which is on the opposite side, to the south east of Athens.
Of course you will still have to take a taxi to pick up the car (twice cheaper than a taxi to the airport)
I guess that the OP could arrive in Athens from Nantes, France with a Volotea flight.....