What is the traffic like on a Monday evening driving from the Airport into Athens?
Traffic is always heavy in and around Athens. And if you don't know the street layout it will be very difficult to find your way around. Make sure your car has an updated GPS navigation system.
As my mother used to say at the bridge table -- can we have a recap of the bidding? You posted previously about getting a van transfer to central Athens for your family of 5, then a car rental there to go to the Peloponnese. Now u plan to drive into Athens? Just curious; do you save enough on airport vehicle pick-up to pay for overnight parking in central Athens? My Greek friends say it's $$$ -- but maybe your rental place has a garage. Traffic isn't terrible after 7 pm, but confusing unless your apt is right off a main street.
I haven't had any troubles parking in Athens.
When I am working leading tours I park my vans in a lot for 10 Euros / night.
When travelling alone the hotel I stay at parks my car for me somewhere in the neighborhood. My old hotel had on-street parking all around.