Delphi was the sacred center of the world for the Ancient Greeks and undoubtedly a remarkable achievement of purposeful architecture, a sanctuary built into the challenging landscape to inspire awe and worship. Unfortunately, very little is left of its former glory, and the visual impact is fairly muted. Having experienced grandiosity of the Parthenon at Acropolis or the vastness in various states of preservation at Delos, it is hard to see Delphi as little more than a sequence of terraces with few surviving features here or there. Of course, you may be more into archaeology than I am or simply more attuned to the aura of the great legacy that the Ancient Greece bestowed on the mankind, so the simple fact of being in such an important and symbolic place may give you additional satisfaction.
The focal point of Delphi is the Temple of Apollo, undoubtedly photogenic, backed by the views over the valley of Phocis. Among other defined points are the Athenian Treasury, the theater, and the stadium up the mountain. The higher you climb, the more sweeping the views become, but in reality climbing more than a terrace or two above the Temple of Apollo is worthwhile only if you are looking for some mountain hiking.
Delphi is located in the administrative region of Central Greece about two and a half hours drive from Athens. An hour appears sufficient to explore the core of the site (for aforementioned hiking, add another hour). The archaeological museum is a short walk from the site along the main road and is accessible for a separate fee (or on a combo ticket); allow one more hour if you want to view its collection.