There is also the weekend parking lot for the "new" Air Force Memorial (those three spires visible behind the Pentagon). The view from there should be stunning, with the Pentagon, the Mall and its Memorials and the Capitol in the background, across the Potomac.
But I have to go there one of these coming weekends to scout the place (possible clearance, weekdays limitations for sure since it's used by Pentagon's staff...). I'll keep you posted.