We had something else planned this afternoon so we couldn't do anything that took too long or that might have gotten us into a pickle so we decided to go to see the old mud brick ruins of Raghba. I wasn't able to find a lot of information about the place, but it's supposed to have been around since the mid-1600s. There used to be watchtower on the west side of the ruins (as recently as a couple of years ago), but it was a heap today.
You can still go inside the mosque and climb up the minaret to get a view over the ruins. Don't miss that. There are also some buildings with some interesting decoration inside. You could poke around diligently for a couple of hours, but I'd suspect most people wouldn't stay more than an hour at most.
It's easy to get here from Riyadh. Go northwest out of the city on King Khaled Road (535) and just keep on going till you get to Raghba. There are plenty of signs along the way in Arabic and English. It's about an hour from the time you leave the edge of the city. When you get to the circle at Raghba, the ruins will be to your left. Turn left at the circle and drive down into them.