Masmak Fort & Old Riyadh
Masmak Fort (Qaṣr al-Maṣmak) was the centre of old Riyadh and something of the symbolic birthplace of the modern Saudi state. Today it's a beautifully restored and illuminated museum, chronicling Abdulaziz bin Abdul Rahman Al Saud's early 20th century unification of the Arabian peninsula for the first time in a millennium. It's also free, which is always nice, and all the more so in a city that's generally not cheap.
While the Saudi Arabian Coffee exhibition was both well-presented and informative, for us the highlight of the place were the old photographs of Riyadh, which look like they were taken during pre-camera era of the First Saudi state in the early 18th century, but most of which are actually from the 1950s - ie that decade when the Soviets were already launching dogs into space for fun (RIP Laika). What a difference a few decades and a few (hundred billion) barrels of oil make.....