During the summer months the Acropolis suffers from too much heat and too many visitors. Just about every tourist will visit the Acropolis at least once during their holiday, so the sanctuary sees a lot more traffic than any other archaeological site in Greece. If you can avoid visiting during the summer months you will have a much more pleasant experience.
But if you are in Greece during the summer, don't worry, a few simple tips can help you enjoy your visit. The best time to visit is before 9:00 am, and even then in the summer you might find yourself in the midst of large crowds of people. The crowds swell to uncomfortable levels and the heat becomes unbearable after 10:00 am. An afternoon visit might be a lot more comfortable, but remember that the guards begin herding the visitors toward the exit about half hour before the site closes, so get there at least 90 minutes before closing time, and plan on spending at least 45min to examine the monuments comfortably.
Buy your water bottles before you reach the ticket counter since the two vending machines (the only place to buy bottled water at the Acropolis) are often out of order, and when they work, the lines are long. Large bags and coats can be checked at the little building near the entrance, to the left of the ticket counter.
A water fountain, and toilets can be found in the vicinity of the ticket kiosks at the entrance, and once you reach the top of the hill at the East end of the Parthenon. There is a post office kiosk, and a museum shop next to the ticket counter at the south end of the rock.
Tickets are sold at the two gates to the Acropolis. If you reach the site via metro, the ticket kiosk is across from the Acropolis Museum. If you walk to the Acropolis from Monastiraki or Thesion metro stations, you will find the ticket booth at the end of your walk right next to the gift shop.
Tickets to the Acropolis cost 12 Euro each and allow entrance to all the major monuments of Athens that you can visit on foot. The ticket is valid for entry in: Ancient Agora, South slope of the Acropolis (Theater of Dionysos), North slope of the Acropolis, Roman Agora, Kerameikos, Temple of Olympian Zeus, and Hadrian's Library. You may choose to visit these sites at different days with the same ticket, but if you are in a hurry the tour can be done in one busy day. Reduced ticket price is 6 Euro, and children enter free.
The approach to the Acropolis is possible from two different gates at the foot of the rock where ticket kiosks provide entry. Three different train stations surround the Acropolis: Thesion, Monastiraki, and Acropolis. The Acropolis station is right outside the new Acropolis museum and perhaps the closest to the ticket booth for the archaeological site.
One entrance is on the south end, and can be used by those who ascent on foot from Monastiraki or Thisio train stations. The walk from Monastiraki is the steepest, while the one from Thisio is a bit easier. Once you get your ticket you can move straight up towards the Propylaea (the actual entrance to the top of the Acropolis rock) or you can walk the perimeter towards the Theater of Dionysos. You will have to return the same way to reach the Propylaea again.
The other entrance is near the "Acropolis" metro station, right outside the new Acropolis museum. Once you get your ticket, you can move through the south slope of the rock, past the Theater of Dionysos and towards the Propylaea. In both cases, the hike to the Propylaea is fairly steep but the main path is paved with cement and not difficult to walk through.
Once you reach the Propylaea you have to walk up a zig-zag entryway that is very steep and slippery. This often the place where you might find yourself walking through a mass of people through a narrow and precarious path. This zigzag approach is a Roman addition to the Acropolis. The Ancient Greeks used a straight inclining ramp.
As you walk up toward the propylaea, to your left you will notice a pedestal of Agrippa's monument, and to your right the small temple of Athena Nike that seems to be under perpetual reconstruction that has exceeded ten years.
Once you pass through the imposing Doric columns of the Propylaea -- the same ones the ancient people passed through -- you will be free to roam around atop the acropolis. The ground is relatively level, but not necessarily easy to move around since it is either too rough or too slippery in areas.
The area around the temple of Athena Nike is not accessible. While this little temple is completely encased in a metal scaffold, both the Propylaea and the Parthenon have multiple parts wrapped with metal scaffold as well. In fact, three of the four major buildings that are still standing are obscured by excessive scaffold structures and cranes that exist as long as anyone remembers. The Erechtheion, the building with the Karyatides (standing women) holding the porch roof, has been going through its own reconstruction, but the restoration aids have been less distracting.
You will exit the Acropolis through the same gate (the Propylaea) where you entered, so walking in a big circle to your right as you enter the plateau would be the most efficient way of touring the rock. At the northeast end there is an elevated platform where you can get an elevated view of the Parthenon and superb views of Athens all around.