I visited Athens in late September, 2008. It was a great time to visit; temperatures were very pleasant and the big crowds of summer were gone.  The food was great and the sights were fabulous! The ruins are older than Rome and concentrated in one spot (the Acropolis).
The Temple of Olympian Zeus, located just a couple of blocks from the base of the Acropolis. One of the largest temples in the world at it's time. Lots of columns!
The Acropolis. Can't visit Athens without spending a day up here.
Heading up the Acropolis, after paying 12 euros. First is the  Temple of Dionysos.
The Temple of Asclepius, god of health, and his daughter Hygieia, where our word hygiene originates.
The Odeon of Herodes Atticus, an amphitheater built in 161 AD. It seats 5,000 and hosts modern performances now.
Almost to the top. The Temple of Olympian Zeus is in the background. It must have been huge!
The Theater of Dionysos from above.
And waiting for you at the top, is The Parthenon.
The south porch of the Erectheum, containing the six caryatides, female maidens instead of columns. Zeus, Athena, Poseidon and Hermes were worshipped in the Erectheum.
Also at the top, an unusual multi-leveled temple, the Erectheum.
Me on the north porch.
The Temple of Asclepius from above.
After descending the Acropolis, it is a short walk to the Temple of Hephaestus, the patron god of metal working. Like most of the temples on and around the Acropolis, it was built around 440 B.C. The frieze depicts the battle of Centaurs and Lapiths.
The Library of Hadrian, built in 132 A.D. Located next to the Monastiraki metro station, it's in a vibrant part of the city packed with shops and restaurants.
Next I visited Delphi, located in the mountains about 2 hours north of Athens. Delphi was populated at least 1600 B.C., and contains a stadium for the predecessors of the Olympic games, which were held every 8 years. Regarded as the center of the world and the dwelling place of Apollo, pilgrims came from around the ancient world to seek guidance from the Oracle of Delphi.
The Treasury of Athenians
The Temple of Apollo
The Theater of Delphi, built around 400 B.C. seated 5000.
...sights around Delphi...
My favorite structure at Delphi is the Tholos, located about half mile outside the main entrance to the ruins of Delphi. It must have been a magnificent structure when completed. It's purpose remains a mystery.
The Acropolis from the top of Lycabettus hill. A great view of Athens from the top.
The Oracle answered questions from a small room inside the temple.