It is a very popular hill station, some 1 and half hour drive from Islamabad. I recommend it because Islamabad, during summers, can become hot and bad. So you need to run away from that heat. Murree is a fine option. NOTE: Avoid going to Murree during summer holidays as it is full and dirty. So many people rush there that its usefulness becomes void.
It is 1.5 hour drive from Islamabad.