That's not a bad idea, WWanderer. There is basically 1 car-rental place there, Pop's Cars, on the mainland side of Poros. You can get a 3-day rental for a 4-door automatic shift for 160€. It would avoid the whole business of driving in the Athens area. You could go directly from airport to PIraeus on the X96 bus (75 minutes? 3.2€), get your ferry to Poros --- they go hourly and take about 60-80 mins. Then, off the ferry, into one of Pop's cars and zoooom. Or as ww says, stop for a night, THEN get a car. SUCH a good idea!

Whatever you decide be sure you have a car for Nafplion... getting to Mycenae, Tiryns, Epidaurus etc will be so easy on your own. Ditto Delphi.

If time is short, you can see Mycenae, Tiryns
and Epidaurus in a day. I did it. It wasn't ideal and we only had time for lunch by the water in Nafplion but you have to work with the time you have available.

Don't be daunted about driving in Athens. Spending time in the city with a car can be a hassle but picking up a car and driving out of the city is no big deal. I just drove in and out of Toronto this past weekend and it's no worse than the last time I drove in Athens in 2008! LOL Actually, I think I prefer driving in Athens!

Rob