In Lahore there is the Badshahi Mosque and the Fort. Then Shalimar Gardens, Bagh-e-Jinnah, (formerly Lawrence Gardens) Jahangir's Tomb. Then Islamabad is a good place with the Faisal Mosque. Not far from Islamabad is Murree. Khewra Salt Mines.
This is my Hub on it.
There are many more places to see in Lahore. I was in Pakistan twice and visited Rawalpindi, Islamabad, Murree, Faisalabad, Khewra and Abbottabad. As long as you keep away from unsettled parts of the country there is no problem travelling in Pakistan. Just be careful about where you eat and always drink bottled water. These are the main things because you cannot do much if you are sick. You can get very good accommodation for £25 per day and in the hills like Murree or Abbottabad I have paid about 500 rupees in 2005. If you want to visit just do some research and it is better if you travel with someone but other than that, have no worries and enjoy yourself.