So much depends on what you are interested in . . . how much you go to Greece for archeological/historic sites. The Agora is just below the Acropolis and was the main gathering place for ancient Greeks, business, government, shopping, chariot racing, debating, and so much more.
I have been to the Agora several times and still go back. It's quite an experience to wander around the ground that Socrates, Aristotle, Pericles and so many ancient Greeks walked. While there is not a whole lot left to see other than foundations, walls, and a few columns here or there the Temple to Hephaestus is in even better condtion than the Parthenon on the Acropolis. It's just a very lovely area, quiet, peaceful and you can almost forget you are in a city of millions.
You can spend an hour, two hours, or many hours just wandering around. Don't overlook the re-created Stoa that will give you an idea of what it was like 2,500 years ago. Very cool and relaxing to step inside out of the hot sun. Nice museum and a good place to hang out, catch your breath and then move on.
A once-in-a-lifetime place to spend time and a good way to learn about Ancient Greece!