Perhaps the most incredible site of any Greek site, the Valley of the Temples — and not really a valley at all. The most iconic temples stretch up a rocky spur, but at the base of the spur is a collection of temples, including the massive Temple of Olympian Zeus, almost entirely in ruins — but enough remains to see that it was over 100m long and was built in a rather novel way, including telamones (huge human figures serving as columns) in some part.
The (certainly misnamed) Temple of Concord:
The Temple of Hera:
A telamon from the Temple of Olympian Zeus:
The ornamental fishpond, built by Theron with prisoner of war labor, now a citrus grove:
There is also a breath-taking museum.