You need somebody in the armed forces. They will book the place for you and then you just go there and pay the rent. For a normal room in the officers' mess the rent for service men is around Rs.600 and for civi guests is around Rs.1400. You can get a suite too by paying more.
PAF's Kalabagh base has some nice suites, pretty place, a small ski slope above inside the base and its a 20 minute drive from Nathiagali. Its a nice, quiet, small town and doesn't have the tourist hordes that the other towns to the east have. At the same time all the other towns in the Galliyat and Murree (belgh!) are easily and quickly accessible from here.
Then there is a small army hut in Thandiani hilltop resort. This is an even prettier place, an hour or so away from Abottabad. You get gorgeous views of the Kashmiri mountains on one side of the town and the Kaghan mountains on the other. This, however, is a remote area with no other towns of note nearby. But you can get to Nathiagali or Abottabada form here within an hour and a half.
For the rest of the Galiyat I would recommend ditching the army rest house and going for the PTDC motels. But you would have to make reservations way in advance. Contact the people there to know what's the latest you can book a place. The places I would recommend would be Nathiagali or Ayubiya. Nathiagali will be more peaceful than Ayubiya, cleaner too. Murree, I don't even like going to, let alone staying at. Its been pretty much trashed for quite some while now.
There's a PTDC motel in Bhesham too. Its a couple hours or so further from Mansehra on the KKH. Pretty enough and very peaceful.
In Kaghan/Naran, again I would strongly recommend the PTDC motel in Naran. It's built right on the banks of the Kunhar river, across a small bridge over the stream coming from the Saifulmaluk, beyond the north north-east edge of the town of Naran. This place I would recommend you over all the others near Islamabad (Galliyat, Murree, Abottabad, Bhesham, etc.). The town of Naran has recently been inundated with tourists but they keep themselves between Shogran and the town of Naran and then up to the Saifulmaluk. Starting from the PTDC motel up till the Babusar Pass the places are virtually untouched from annoying tourists and spectacularly beautiful. The road beyond Naran itself is something to be experienced and enjoyed. There are quite a few lakes beyond Naran too, e.g. Lulusar, Dudipat, etc. Gorgeous places. Plenty of angling opportunities for rainbow trout (you'll need an inexpensive permit which you can get in 15 minutes from Naran), small islands in the middle of the Kunhar river with tea huts, tens of easy trekking routes which you can explore, and much more.
But if you decide that you wouldn't mind going further up north, let me know. I have a brilliant little hut in mind. Ranjit Singh used to spend his summers there. The place was restored by the army but can be booked by a civi as well.