The Acropolis is a flat-topped rock of about eight acres that rises about 400 feet above the center of modern Athens, today a city of about four million people. The earliest artifacts found on the Acropolis date to about 10,000 BC. Two of its most well-known structures, the majestic Parthenon and the elegant Temple of Athena Nike, were built during the Golden Age of Athens in the 4th and 5th centuries BC.
I highly recommend, once the outer gates open at 9am, a morning walk up the southern side of the Acropolis, climbing above two open-air theaters -- one ancient Greek, the other Roman-era -- past the olive trees, to arrive on foot, as people have for centuries and centuries, at the Propylaea entrance. You won't avoid the crowds, but it's a wonderful experience.
Another recommendation includes getting lost in the lower hillsides on the north and east sides of the Acropolis, where centuries of homes, churches and a monastery or two are connected by only narrow and tumbling walk streets. Nearby is the handsome and spaciously modern Acropolis Museum. http://en.wikipedia.org/wiki/Athens