Hagia Sophia

Pros

• Major attractions such as Hagia Sophia, Basilica Cistern, Topkapı Palace, Sultan Ahmed Mosque, Galata Tower, and Maiden's Tower are all impressive, though entrance fees are relatively expensive (around €20–30 per site).
• Hagia Sophia is really cool too right next door.
• Istanbul highlights (first-timers): • Hagia Sophia, Blue Mosque, Topkapi Palace
• Hagia Sophia, Blue Mosque, Basilica Cistern/Galata/Topaki Palace

Cons

• Tickets to some of the attractions are frankly now a bit ridiculous in Istanbul with tickets to Hagia Sophia, the Cisterns and some others being around $40USD+.
• For how little time you have, i honestly wouldn't recommend entering any paid attraction like the Hagia Sophia or Galata Tower (i wouln't enter these two even if i had more time) or the Topkapi Palace.
• Unfortunately the Hagia Sophia do not issue refunds and the airline won't issue one as they see Turkey as unaffected so there's a sunk cost factor.
• I would really recommend not forking over the insane ticket fees for the Galata Tower and even Hagia Sophia. There’s better options for free.
• Hagia Sophia is still impressive despite ongoing renovations, but signage can be confusing and crowds are heavy.