I am planning on renting a car to explore areas outside of Athens and really want to avoid driving in Athens if at all possible. I plan to head to Peloponnesia via Corinth first and loop around and return via Delphi. Looking at the maps I cannot tell how hard it will be to avoid driving in Athens if I rent at the airport. I know Rick mentions a rental company that will drive you out of the city--but i would prefer not to drive it back in to the city.
Beginning at the airport, if you stick to the National Highway E94 you will skirt around Athens altogether all the way to the Peloponnese. Look here: http://tinyurl.com/omlmuat
Just did this exact thing last week...no trouble avoiding Athens. The highway signs are clear and our iPhone navigation maps worked fine (although spelling alternatives for city names are tricky). Be prepared for toll booths...most are less than €3, but the bridge from Peloponnesia to Delphi is (about) €14.