Istanbul is so historically significant that it gives you goose bumps. You can stand in the huge cupola room of the Hagia Sophia and imagine the Emperor of Rome entering through the massive door that only he can use. You can pause under the dome of the Blue Mosque and imagine how hundreds of craftsmen work on the intricate tile patterns.
One of the most inspiring places in Istanbul is hidden. Below the Sultanahmet district there is a huge underground warehouse. Your boat meanders through a forest of columns and moves deeper into the cave space. It is a miracle of old technology and an atmospheric experience.