No trip to Istanbul would be complete without a trip to the old city including a visit to the Sultan Ahmed Mosque (Blue Mosque). It was constructed starting in 1600, completed some 16 years later.
Overall it is huge, 6 minarets surround the main dome. The courtyard is almost as big as the mosque itself. The interior is visually stunning as well as very peaceful given all the visitors milling about.Read about the history before you visit.