This is what I would do. Make it an enjoyable journey. Hire a car in Athens and drive to
Palaia Epidavros where you can have lunch and go to the
nearby beach to swim/snorkel amongst the ancient ruins. Then go to Nafplio for the evening for dinner and stay overnight. The next day you can climb up to the
Akronafplia Castle then drive to Monemvasia through Leonidio which is a beautiful town with climbing mountains nearby. After a couple of nights at Monemvasia, take the ferry to Kythira from
Neapoli. After your stay in Kythera you might want to come back to Neapoli and do a drive inland going through Sparta which has an amazing ancient city. There is so much to see on Greece's mainland and much that I haven't even mentioned like
Mycenae and many other ancient sites. So much to do and see in Greece! Whatever you decide to do, you will enjoy it!