Entrance to the site and museum will cost 6€, unless this was included in the price of your excursion. At the ticket kiosk, you'll see a number of site plans and picture guides for sale; Delos & Mykonos: A Guide to the History and Archaeology, by Konstantinos Tsakos (Hesperos Editions), is a reliable guide to the site and the museum. Signs throughout the site are in Greek and French (the French have excavated here since the late 19th c.).
To the left (north) of the jetty where your boat will dock is the Sacred Harbor, now too silted for use, where pilgrims, merchants, and sailors from throughout the Mediterranean used to land. The commercial importance of the island in ancient times was due to its harbor, which made Delos an excellent stopping-off point between mainland Greece and its colonies and trading partners in Asia Minor.
The remains at Delos are not easy to decipher, but when you come ashore, you can head right, toward the theater and residential area, or left, to the more public area of ancient Delos, the famous avenue of the lions, and the museum. Let's head left, toward the Agora of the Competialists, built in the 2nd century B.C., when the island was a bustling free port under Rome. The agora's name comes from the lares competales, who were minor "crossroad" deities associated with the Greek god Hermes, patron of travelers and commerce. This made them popular deities with the Roman citizens here, mostly former slaves engaged in commerce. The agora dates from the period when Delos's importance as a port had superseded its importance as a religious sanctuary. To reach the earlier religious sanctuary, take the Sacred Way -- once lined with statues and votive monuments -- north from the agora toward the Sanctuary of Apollo. Although the entire island was sacred to Apollo, during Greek antiquity this was where his sanctuary and temples were. By retracing the steps of ancient pilgrims along it, you will pass the scant remains of several temples (most of the stone from the site was taken away for buildings on neighboring islands, including Naxos, Mykonos, and Tinos). At the far end of the Sacred Way was the Propylaea, a marble gateway that led into the sanctuary. As at Delphi and Olympia, the sanctuary here on Delos would have been chockablock with temples, altars, statues, and votive offerings. You can see some of what remains in the museum, which has finds from the various excavations on the island. Admission to the museum is included in the site's entrance fee. Beside the museum, the remains of the Sanctuary of Dionysos are usually identifiable by the crowd snapping shots of the display of marble phalluses, many on tall plinths.
North of the museum and the adjacent tourist pavilion is the Sacred Lake, where swans credited with powers of uttering oracles once swam. The lake is now little more than a dusty indentation most of the year, surrounded by a low wall. Beyond it is the famous Avenue of the Lions, made of Naxian marble and erected in the 7th century B.C. There were originally at least nine lions. One was taken away to Venice in the 17th century and now stands before the arsenali there. The whereabouts of the others lost in antiquity remains a mystery; five were carted off to the museum for restoration some years ago and replaced by replicas. Beyond the lake to the northeast is the large square courtyard of the gymnasium and the long narrow stadium, where the athletic competitions of the Delian Games were held.
If you stroll back along the Sacred Way to the harbor, you can head next to the Maritime quarter, a residential area with the remains of houses from the Hellenistic and Roman eras, when the island reached its peak in wealth and prestige. Several houses contain brilliant mosaics, and in most houses the cistern and sewer systems can be seen. Among the numerous small dwellings are several magnificent villas, built around a central court and connected to the street by a narrow passage. The mosaics in the palace courtyards are particularly dazzling, and include such famous images as Dionysos riding a panther in the House of the Masks, and a similar depiction in the House of Dionysos. Farther to the south is the massive Theater, which seated 5,500 people and was the site of choral competitions during the Delian Festivals, an event held every 4 years that included athletic competitions in addition to musical contests. You may see donation boxes here: the Diazoma Organization, fearing that the Greek government has neither the funds nor the will to restore the theater, is attempting to finance the restoration privately. Behind the theater is a fine arched cistern, which was the water supply for the city. If you visit here in spring, the wildflowers are especially beautiful, and the chorus of frogs that live in and around the cisterns will be at its peak.
If you're not on a tour and have the energy, consider wrapping up your visit by getting an overview of the site -- and of the Cyclades -- from Mount Kinthos, the highest point on the island. On many days, nearby Mykonos, as well as Siros to the west, Tinos to the north, and Naxos and Paros to the south are easy to spot. On your way down, keep an eye out for the Grotto of Hercules, a small temple built into a natural crevice in the mountainside -- the roof is formed of massive granite slabs held up by their own enormous weight.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.