Islamabad is truly a capital city, and because of this you'll find that they don't really have local souvenirs. Instead Islamabad has souvenirs from all over Pakistan with a huge variety to choose from. If you're going to be traveling outside of the capital you may want to wait for certain souvenirs, as in their local regions they're often cheaper and offer more variety. Many regions of Pakistan are famous for certain products or trades, salt from the salt plains, hats from Gilgit, etc.
The great thing the capital offers is the chance to buy things from areas you may not visit. Many items might be made in Islamabad, but in the traditional way from the region where it is famous, and often made by someone from that region. You will likely pay a premium for these souvenirs in Islamabad compared to what you might pay in their original village or region, but it might be the only way to get certain items.
There are many different types of local arts and crafts available in these gift shops. Local things like hand made pashminas, wooden furniture, and all kinds of brass art just to name a few. The truck art section of any gift shop is also worth checking out. Carpets are very popular souvenirs however many are imported from outside Pakistan, so while it's a good buy, it's likely not Pakistani.
No matter what you buy, just make make sure to bargain as the suggested price is never the best price. Also, feel free to look around as there are multiple dealers selling just about everything.