Take in the grandeur of Ecumenical Patriarchate of Constantinople. Schedule in a service if you can or just wander around and appreciate a break from the outside world.
This Istanbul City Center principal church is an interesting spot to begin your day of sightseeing. There's plenty to see and do close by, including Blue Mosque, Hagia Sophia, and Topkapi Palace. Take your time at Ecumenical Patriarchate of Constantinople before making tracks to these other fine attractions.