Wikipedia :Ruins of the Temple of Hera  ;

The oldest temple at Olympia and one of the most-venerable in all Greece was that of Hera, originally a joint temple of Hera and Zeus until a separate temple was built for him. 

From Britannica :Ruins of the Philippeion The Philippeum, a circular building of the Ionic order, with Corinthian half columns on the inside, was begun by Philip II (Philip of Macedon) and probably finished by his son, Alexander the Great. Containing gold and ivory statues of Philip, Alexander, and other members of the family, it commemorated Philip’s victory over the Greeks at Chaeronea in 338 bce. 

Crypt (arched way to the stadium) 

The Olympia stadium : ( Text from Britannica )

The stadium : Text from britannica :

The stadium lay to the east of the Altis. In early classical times it was not cut off from the sanctuary, and one end of the track was directly in front of the great ash altar of Zeus (beneath the later Echo Colonnade). About the mid-4th century bce the stadium was shifted some 270 feet (82 metres) eastward. The track was surrounded by massive sloping embankments of earth for the accommodation of the spectators (some 40,000), except to the north where the natural slope of the hill sufficed. The western embankment, parallel to which the Echo Colonnade was built, effectively cut the stadium off from the Altis. Connection between the two areas was maintained by what was called the Krypte, or Covered Entrance, which pierced the embankment and, in Roman times, was covered with a stone vault. This entrance was used by the athletes and the umpires. Just outside the Krypte stood bronze statues of Zeus, called Zanes; they were erected with money from fines imposed on those who violated the rules of the Games. The bases of 16 of these statues have been excavated.

The only stone seats in the stadium were located in a box on the south side, about one-third of the way from the starting line nearest the Altis; there sat the hellanodikai, the chief judges of the Games. Directly opposite the box was the altar of Demeter Chamyne, from which the priestess of that cult was privileged to watch the Games (sources generally agree that other women were excluded from the festival). The track area was about 690 feet (210 metres) long and 105 feet (32 metres) wide and separated from the sloping embankments by a low stone parapet beside which ran an open stone water channel with basins at intervals. The stone starting lines at either end of the actual course are still in place, lying about 630 feet (192 metres) apart. There was space for 20 runners at a time. The classic race was the stade—i.e., one length of the course. There were also a diaulos, two lengths, and a dolichos, or long-distance race, the length of which varied and might have been as much as 24 stades, or nearly 3 miles (4.8 km). Other athletic contests were also held in the 4th-century stadium, which has been fully excavated and its track and embankments restored.

When the stadium embankments were excavated, many votive offerings were discovered. Some of these were works of art, including bronze statuettes and reliefs and several terra-cotta statues, of which the most noteworthy was a group of Zeus and Ganymede, about half-life-size and dating from about 470 bce. Other notable votive offerings were arms or armour that had been dedicated in the sanctuary. The Archaeological Museum houses these findings, which constitute the largest collection of ancient Greek weapons in the world. Some of the weapons have identifying inscriptions that are interesting historical documents, such as a Persian helmet marked with the phrase “The Athenians [dedicated the helmet] to Zeus, having taken it from the Medes.”

The hippodrome lay south of the stadium in the open valley of the Alpheus River. Pausanias gives a long description of the hippodrome and of the elaborate machinery used to start the horse races.