That seems like a good plan. The main streets between the Kingdom Tower and the Faisal Tower are the best places to experience Riyadh in such limited time.

The main cultural experience to be aware of is prayer time. It occurs five times per day, including dusk, and is signified by imams singing the call to prayer over loudspeakers from mosques. A Westerner shouldn't be out in public during that time. The response will range from dirty looks to getting shoo-ed by the religious police. Just keep a low profile -- leave a retail establishment if you're in one and camp out somewhere nondescript. (I was in the mall in the Kingdom Tower once, and just sat on some chairs in the corner outside of Starbucks.) Most restaurants don't open until after the evening prayer, but one good option is to be seated before then, and then when the time comes they just close the doors to the restaurant and you can continue.

Not a huge deal, just something to be aware of, and an interesting cultural experience for the visitor.