Temple of Olympian Zeus
Huge, 5-story columns and the Arch of Hadrian, as you enter the park. Once was the largest such temple in the known world.
(Elsewhere called Philopappou Hill.) Monument on the top of the hill can't compare to the nearby Acropolis, but the view from the top is wonderful. Also, the jail cell where Socrates died (maybe) is on the way up. It's only a 7-minute walk from the Acropolis--do it.