Ken, I see u are from Wyoming, which has about 1 person per square mile, right? However I trust you've driven in busy areas too. Don't let T of seattle scare you... but his advice is good;
• why you need a good map besides GPS: sometimes GPS gets confused ... there may be 2 towns with same name in adjoining areas (like the Springfields all over USA).
• Avoid Athens driving; its like trying to get out of Manhattan to New England when you don't know the routes. The Athens agency Swift/Avanti gets lots of business by driving you from hotel directly to the on-ramp of the intercity highway; just be sure to specify a new or nearly-new model car -- some people report getting older cars w. unreliable AC. If u rent from airport, lots of choice at good prices.
• Highways -- the highway almost all the way to Nafplio is THE most modern in Greece & 2 lanes each way w. central divider. When u get off on the road to nafplio, its 1 lane each way, but very little traffic in May; that's when I've used it. And the roads around Nafplio are narrow but again, very little traffic; you can slow down &enjoy the views. The Road to Delphi is 4-lane for about half-way, then 2 lane as you get into foothills, but again not much traffic in May.
• One thing for sure, tho -- stay ALERT for turn-offs (my role has been the navigator so I'm aware). In GReece, there's first a sign in Greek, then about 500 yards on , a sign in English (yes!) BUT USA roads cater to slow reactors/distracted drivers. Our road signs tell us "1 mile to Springfield" "1/4 mile to Springfield" "Springfield next exit" etc. Greek signs say "Exit to XXXX" about 500 yards before the exit. And that's IT -- no second chances. So don't chat or sightsee when coming up on a turn-off.
RESERVING -- Since you can drive a manual transmission, you'll get cheaper rates, and will not have to reserve way way in advance, if using an airport agency; this one -- http://www.athens-carrental.com -- is highly recommended by a UK forum "regular" who rents every year. Even if you don't rent from them, their very comprehensive price-list onsite can help you comparison-shop. For Avanti/Swift, best to rent farther in advance because of smaller fleet.
Then there's the usual rental advice:
• You need an IDP (International Driving Permit) to rent a car; NOT a license; you just stop by any AAA office & get one in 15 minutes; costs about $20.
• When picking up car, or getting it delivered to hotel; do a walk-around to note ANY dents or other dings, take a photo, and have it noted on the contract you sign.
• Even tho your credit card offers you insurance, take the agency's full CDW, its worth if for peace of mind -- if by any chance you get in a fender-bender, it can be a hassle to contact your home agency with a 9-10 hour time difference.
A final note: As for Delphi, I'm glad to see you're not rushing it for a day trip... but it can be done as a 1-night, with good timing. Driving there takes about 3+ hours... if you arrive in late afternoon, you can check into hotel, then walk along the road back to the MUSEUM, it's open until 7:30 or so, and NO crowds... you can also walk downhill to the Athena Tholos ruin (the round one), it's open24/7 and lovely at sunset. Then go have a nice dinner looking down the mountain. Next AM be at the Ruins site at 8:15 opening... and you'll have over 2 hours with almost no one on the site, until tour busses arrive. Then you can leave by noon or so.
For the Peloponnese, here's a great online map showing all roads large & small & landmarks - http://euro-map.com/karty-grecii/peloponnes/podrobnaya-turisticheskaya-karta-peloponnesa.jpg -- click & it gets huuuge.
Happy planning! You've chosen a wonderful time to visit greece -- the rains are over but the greenery is lush, flowers abound, the sea is "brisk" but swimmable, everything's open, nothing's crowded.