Established in 972, Al-Azhar represents the oldest mosque and the oldest operating university in Cairo with more than thousand years of history behind it. While the exterior is notable primarily for its minarets that were added to the building during Mamluk dynasty, the interior is worth visiting for its beautifully decorated marble courtyard.
Admission is free.
Expect to tip the men looking after your shoes upon your return.
Wikipedia https://en.wikipedia.org/wiki/Al-Azhar_Mosque
Address 4 Haret El Elaish, Cairo, Egypt
Coordinates 30°2'44.444" N 31°15'45.669" E