Murree during EID and 14 aug holidays is nothing more than a traffic jam...and not only Murree itself but the entire road from Islamabad to Murree.
There has been instances where people left for Murree on Eid holidays with family and had to sleep the night on the road and come back without even reaching there. At other points, the police simply block the road to Murree because there is already too much traffic than the place can handle, not to talk about sky-high prices of hotels and restaurants if you even find one.
Avoid Murree at all costs and you can thak me later!
In Islamabad, you can visit Faisal Masjid, Shakarparian, National Monument, Zoo, Rawal lake view point, Daman-e-koh and even Pir Sohawa. However, Daman-e-koh and Pir Sohawa will also be a traffic jam with no gurantee you can even reach there....depends on the day and weather.
If you must go somewhere, try Banjosa lake in AJK or River Kunhar near Balakot. Both are doable as day trips (with 4 hr drive one way).