Kiffa has a nice market and is on your way from Nouakchott to Nema. It makes for a nice stop, although there is no hotel in town but a nice bed and breakfast on the road when you arrive from Nouakchott. Nearby is an interesting canyon you can visit. For this youll need an all-terrain vehicle.
Kiffa has quite a reputation nas a centre for beads. So if you are here, you could go to the market and get you some.