There a busesalmost every hour to Nafplio. Take an early one and you will have virtually the whole day in Nafplio. This is a very inexpensive way to do Nafplio. The old town central area plus the two Venetian fortresses Palamidi and Bortuzzi, will take you a day.
Nafplio and Peloponnese http://www.flickr.com/photos/stanbr54/sets/72157632094108982/
If you decide to rent a car you have to pass Corinth so stop at the canal, Ancient Corinth and although no one has mentioned it, Ancient Nemia which had sister games to Olympia is also right there about a two minute drive off of the National road. Nemia still has its stadium. Total time to visit Corinth sites and drive to Nafplio probably 4 to 5 hours so you will still have the afternoon for Nafplio.
By the way when we rent a car we take the Metro to the airport and rent there./ This means you are on the National road immediately and do not have to drive in Athens.
Images of all this in the album above.
Oh yes I recently read that Nemia is closed for the winter so if you are doing this trip before May there is no point in stopping at Nemia.