Your tips Pay a visit to the Basilica Cistern. This is the largest of several hundred ancient cisterns that lie beneath the city of Istanbul. It is located 500 meters from the Hagia Sofia.
Istanbul is a great city to spend some days. A visit to the Hagia Sofia and the Blue Mosque is recommendable (be properly dressed for the Blue Mosque!!). Avoid the touristic places when eating out. The small, more obscure looking restaurants in the side streets serve far better and authentic food than the big tourist restaurants.