Where to go out of Islamabad?

Well, I guess the simple alternative would be to pick up an Indian visa and head up to the Himalayas, possibly Srinigar and on to Ladakh would be the obvious route.
UB.

Thanks baba
That was my first thought too, even though we travelled & trekked in that area last year (from Leh to Srinigar).
Other opitons?

Nepal would be another option.

I dont think that there are direct flights from Islamabad to Kathmandu, so you probably have to fly via Karachi. Assuming that you are flying with Emirates, you might be able to reroute your flight (change the destination from Islamabad to Karachi or Delhi, and from there fly to Katmandu). Emirates doesnt fly to Kathmandu.

In Nepal of course plenty of options for trekking. I'm not sure how the weather is in july and august.

''In Nepal of course plenty of options for trekking. I'm not sure how the weather is in july and august.'' .......................................Monsoon, hot, humid, wet and generally considered to be the worst time to trek.
UB.

I guess the Ismaili and Shia areas in Northern Pakistan (Hunza, Skardu, ...) are safe enough. Nanga Parbat is in a fundamentalist Sunni area, this area has never been extremely safe even before Islamist terrorism.

Thanks.
We're quietly still considering Skardu. Feeling a bit hesitant though.
Next option will be Ladakh subject to flight connections. We'll probably decide last moment.