1/ visit the Cycladic art museum and the Benaki museum ,walk and have cofeebreak(Filion cafe with its outstanding sweets and ice creams and snaks is excellent), in Kolonaki and then take the funicular up to the Lykabettus hill

2/ take a taxi down to Vouliagmeni's Astir beach, swim,relax and then finish your evening dinning in one of the seaside tavernas like Lambros,or Maistrali.

3/ take a fast boat to Aegina (max 45min ) and even Hydra if the weather is still warm ,walk ,dine and take the last boat back to Athens

You should put the Benaki museum on your list if not already there. Are you interested in greek music, if so the musical instrument museum is very interesting, you can listen to the instruments while learning about the exhibits.
A trip on the metro north to Kiffisia is interesting, it goes past a lot of Olympic venues, Kiffisia has some upmarket shops and some elegant people although I thought the area a bit dilapidated but even so enjoyed it there. That will fill in a few hours for you.

Thank you both for these ideas. Really appreciate them.

Bob

This site might help:

http://www.athenstaxi.net

When you go to Corinth, be sure to go up to the top of the mountain. There are rather well-preserved ruins of a the Fortress of Acrocorinth. Here's what the Greek National Tourist Organization says about it:

The imposing mass of Acrocorinth (Akrokorinthos) rises out of the ground at the edge of the ancient city. At the top of the pinnacle, at an altitude of 515 metres, stood the acropolis of Corinth during ancient and medieval times. This is the largest and oldest fortress in the Peloponnese. The Franco-Byzantine-Venetian fortifications (built atop the ancient walls) and Turkish additions have left deep signs of their successive claims to supremacy.

And the view from up there is spectacular!

If you are going to Corinth, you may want to drive on to Nafplion or Myceneae for the afternoon and then drive back to Athens in the evening. There is also a lovely little beach town called Tolos on the Nafplio Corinth road that we spent a long lazy day at.

When walking through the town of Delphi, pop into #10 Apollonos St. and say "Kalimera" to my good friend and owner of the store "Constantinos". You will be in for a treat. He may even teach you a Greek folk dance if you are lucky.