Hallways, conference rooms, shops (there is a small mall in the Pentagon), more hallways, bunkers, monuments / military museums, and a bunch of other general use areas.
It's important to keep in mind that when the Pentagon was built, a computer was a person who did calculations and documents were sent to the secretarial pool to be typed; so the number and roles of its occupants were quite different. The space has been re-purposed over the past 70 years.
It's a very interesting building. You can tell that it was made with a focus on people walking from place to place. I think there are tours open to the public (but require a reservation). I'm pretty sure it's the only tour in the U.S. that features a Marine walking backwards for over a mile. I think there are over 15 miles of hallways in the Pentagon. There are 5 levels with 5 rings each, so you can imagine that's a lot of places to walk.
One of the things that isn't so nice now is the anti-radiation film that got placed on the windows after 9/11. The film is bright yellow, so everything that sunlight touches has kind of a gross pee colored tinge to it.
