Blue Mosque (also known as Sultan Ahmed Mosque), a must see in Istanbul, Turkey.
I love love love the building. Especially in the night.
View from the roof: "If one had but a single glance to give the world, one should gaze on Istanbul. "
Alphonse de Lamartine
Walk to Istanbul's three-in-one equivalent of the British Museum via the grounds of Topkapi Palace or through Gulhane Park. If time is tight, go straight to the large porticoed building housing the glorious sarcophagus of Alexander which depicts scenes from the life of Alexander the Great in vivid 3D.