Hagia Sofia (Istanbul, Turkey)
In Istanbul, you can see many beautiful Islamic sites (some of which were Christian churches back when it was Constantinople). [It’s Istanbul, not Constantinople …. If you have a date in Constantinople, she’ll be waiting in Istanbul; now it’s stuck in my head].
The most visited site is Hagia Sofia, which has been a worship site for multiple religions over the years, and now is a museum. It was originally built around 500 AD and was an Eastern Orthodox Church until the Crusades, when it became a Catholic Church. After Constantiople fell to the Ottoman empire, it became a Muslim Mosque and then finally a museum.
The Blue Mosque is another popular site in Istanbul. See more about that here.