3 Days...
Neelum Valley, its beautiful.
Day 1 - Leave Islamabad, Lunch in Muzaffarabad, after Lunch go to Pir Chanasi, I think there will be snow on top, go till your car can go, back to Muzaffarabad for night stay.
Day 2 - Leave MZF early go to Karen, night stay. Enjoy the view on the way, rivers, waterfalls, etc.
Day - 3 Leave Karen early go till your car can go, i think you might make it till Sharda, lunch in Sharda and back to Islamabad.