Abby, Doug is right ON! The car does enable you to do a LOT of sightseeing in a short length of time.... and doing it in the right sequence saves backtracking. Here's the BEST (and noncommercial) website about Nafplio AND the surrounding Argolid area: http://www.visitnafplio.com/ -- I URGE you to click on every link... sights, beaches, where to stay, dine, hike etc. And here's a super photo album by another Nafplio fan, showing beautiful Old Town plus ancient sites around -- http://www.flickr.com/photos/stanbr54/sets/72157632094108982/ --
If you need automatic-shift, advance reservation is good, because fleets have more stick-shifts than automatic. Jessica's agency is a good choice. Their lot is off-premises but they meet you with the car at airport, no problem. You will be pleased with the ride to Nafplio -- it's on Greece's most modern divided highway, a "straight shoot" to the Corinth Canal. Some sequence ideas, depending on age of your daughters, interest etc:
• EN ROUTE OUTBOUND -- Just about 15 Km after Corinth Canal, You can exit at "ANCIENT NEMEA" on RH side, about 5 miles, to an "unknown gem" -- a Sacred Games Site (runner-up to Olympia), with temple, small museum AND stadium, w. original starting-blocks for racers! No crowds, u can enjoy in 45 minutes... in nearby Nemea Village on main street there's a bottling-plant/tasting room for the famous Nemean Red grown in nearby vineyards -- my fave Greek red. Afterwards, you cross biggie highway and on other side is the small (low-traffic) road leading to Nafplio.
• NAFPLIO & AREA - The fascinating city itself is worth a full day, but u can do it in half-day "bites", i.e.: DAY 1 afternoon - Museum in square (open til 3); small, superb, a good intro to area .. then up to top of Acronafplio, view from peninsula pinnacle is aces! And/or walk around base of peninsula, have a late-day swim @town beach. DAY 2 - Zip off EARLY to Mycenae by 8, beat the tour crowds, then hit a beach/ TOLO or ASINI. Late-day, wander Nafplio shopping "lanes". DAY 3 - Final look at Nafplio, then head out.
• EN ROUTE, INBOUND - Head up the BACK of PALAMIDI (avoids the 999-step climb!), go to the ramparts for awesome view & pix. Then take road across the "Thumb" of Peloponnese to ANCIENT EPIDAURUS. The Stunning Amphitheatre will be a highlight of your trip (u don't have to spend too much time on the ruins or museum). Then head for the coast road that goes up to Corinth; if it's a hot day, and you have plenty of time, stop at any beach for a swim. When you get to Isthmus, do a 20-minute stop to walk on the Old Bridge over Corinth Canal (SO deep! SO skinny!). After that, jump on that biggie modern highway back to airport. Final tip: if you must return car with full gas-tank, keep eyes peeled, the last petrol station is about 10 Km before the airport. Bon VOyage!