Avis has cars for hire in Islamabad through Travel Walji’s (2870201; www.waljis.com; 10 Khayaban-e-Suhrawardy, Aabpara, Islamabad). A small car (with driver) costs Rs 2300 per day (excluding petrol); a 4WD is Rs 1000 per day plus Rs 14 per km; and a van (seating up to 12 people) costs Rs 1800 per day plus Rs 9 per km. Note that some companies may require a minimum hire time of one week. You’ll usually need a credit card as a deposit.
Another reputable car-hire company is Safder Tours (5563153) in Islamabad. For something cheaper you’ll need to shop around but be sure to check the small print carefully and ensure you get in writing what you’ve been promised (to avoid potential misunderstandings later on).