Everyone obsesses over the main Hagia Sophia but Little Hagia Sophia (Küçük Ayasofya Camii) near the Sea of Marmara has almost identical Byzantine architecture with maybe 5% of the tourists. Built under Emperor Justin I in 536 AD, in the same period as the famous one.

The prayer hall has the same massive dome and ancient columns, plus a peaceful courtyard garden. Free entry, dress code applies (cover shoulders/knees, women need headscarves), open outside prayer times.

Much more spiritual atmosphere than fighting crowds at the tourist magnet.