Without a doubt, bring a great sunblock/sunscreen along with you (if you travel March-September). There are usually SPF 45's that are PABA-free and sweat-resistent. Bring light-weight fabrics that cover your body well, and if you are a woman, have a pashmina scarf to wear around your head.
Don't make the mistake of wasting space in your suitcase by bringing supplies that are readily available in Peshawar. Take Advil or Ibuprofen with you (most common painkiller is just regular Paracetamol which doesn't always work), a pack of good waterproof bandaids, Polysporin, deodarant, and get a sufficient prescription of Ciprofloxacin (to control the diarrhea). A basic supply you may want to purchase once you get there: ORS (Oral rehydration salts such as Gastrolyte), toothpaste, sanitary pads, and shampoos-- yes, your trusty Pantene PRO-V, and Sunsilk are definitely there.
Make sure to have a case for your camera - Peshawar is unbelievably dusty at times. Good batteries are hard to find in large quantities - so bring a pack with you before you get there. Keep in mind that western battery rechargers WILL NOT WORK in Pakistan.
If you plan on sleeping outside or in a home that only has a partial-roof, I highly recommend bringing some ear plugs. There are tons of critters that crawl around a typical home's unroofed area. Don't make the mistake of having one crawl in your ear like I did. Keep in mind Peshawar is surrounded by farms on either side, and when the wind blows westward it brings a ton of flying-ants and other orchard inhabitants sometimes indoors.
One very important point to make is to try and purchase a water-santization drops from your local Travel Medicine Clinic before you depart. This is a very good thing to have handy, especially if there are places where you cannot purchase bottled water. It will rid the water of cryptosporidium, the nasty bacteria that gives most travellers gastro-intestinal problems on their journey.
