Hello Islamabad is not a very big city but quite beautiful and you can easily browse it in a day. I suggest you to browse Youtube first to get hold of your places of interest e.g. Faisal Mosque, Lok Virsa (Cultural Museum), Shakkar Parrian (hills), Food Street, Some beautiful parks and lake. Do try to visit Murree hills and PC Bhurban (an hour drive from Islamabad) .. Have a nice trip
For starters take trip via Monal to Haripur. It quite scenic keeping in mind its a day trip. May be u can stay at Highland country club on the way back.
Islamabad is newest city, however, surrounding areas may attract you to visit like Takila, Murree,Kashmir (Neelem Vally) etc. however, it is best to have guide who is well familiar to you and is of confidence.