So, have a few days in Athens, there is a hotel called The Phiippos www.philipposhotel.gr where we stayed a few years ago that was (then at least!) very reasonable compared to other hotels and we had two adjoining rooms. It's right at the foot of the acropolis so easy to walk up first thing to the Parthenon and then can walk down through the agora (very nice) and on and around Plaka for little shops/stalls/cafes etc. Go to new museum too.
Then get a car and head off. Drive down to Corinth (ancient Corinth) (couple of hours), there are a lot of hotels close to the ancient castle, again walk up really early (it's worth it) or you'll die. Not much else to do there so head off afternoon and go to Nauplion which is a beautiful Venetian town with lots of lovely places to stay, narrow streets, a lovely port where you can take a boat trip, lots of castles to climb to (how fit you will be), a nice museum, and a really nice evening buzz to stroll around in.
Then go to Mystras (couple of hours), near Sparta - do not go there, there's nothing as the Spartans were only into training and fighting so no monuments or anything of interest!, MYstras is a Byzantine village/town that is deserted and beautifully preserved. Again, go early and start at the top and walk down, it's really stunning. It's not on the well-known site trail for some reason, but coaches do turn up, go early and you'll beat them.
I would then go down to the Mani which is the maddest part of Greece in my opinion. Everyone lived in tower houses and had feuds for generations, supposedly the Maniots were the original Mafia (they all went to Italy). Lots there, there is an amazing boutique type hotel at Geromolinas, and you must go to the Caves of Diros where you float through in a boat. I'd go to the bottom of the Mani, to the gate to Hades, because it is a stunning landscape.
Then go up and round to Koroni and Methoni, keep going up and head into the mountains of Arcadia to Stemnitsa, Dimitsana etc which are stunning and hang off the mountains, there is a gorge there where you can raft (float) which is lovely.
Then across again and back down to Monemvassia, which is a Byzantine town completely hidden in a rock. You can't see it from the sea or the land until you walk through the rock entrance (no cars) and there is an amazing tumble of houses, flowers, churches etc. Nice hotels in and at the entrance.
Go down to Elafonnisos for an astonishing beach (five second ferry) and then end of the island of Kythira (which I haven't visited but want to).