The Hagia Sophia was once a church and then a mosque and is now a museum. It is a spectacular example of Byzantine architecture with its massive dome. The museum has a collection of Byzantine art and remains a major landmark of Istanbul.
Topkapi Palace served as the residence of the Ottoman sultans for almost 400 years. The palace has several courtyards and a harem section. Some of the exhibits include Ottoman weaponry, palace garments, and Islamic manuscripts.
The Grand Bazaar is one of the largest covered markets in the world. The market has over 3,000 shops selling souvenirs, ceramics, carpets, jewelry, and more. The market also has several eateries.
If you have more time, you can visit the Blue Mosque, the Basilica Cistern, or take a Bosphorus cruise. If you have less time, you may shorten your visit to Topkapi Palace or the Grand Bazaar. You can hire a guide who can help you navigate through the city. Taxis in Istanbul are relatively cheap, or use the tram system which connects the areas covered in the itinerary.