North of the Ben Yousef's mosque and North of the bazaars are the Marrakech tanneries. Although you may find many "unofficial" guides who will offer you a tour to the tanneries, you'd better go there on your own.
The Tanneries in Marrakech are of no interest, specially if you plan to visit Fez, but you can drop by if you have plenty of time. Be warned that the tanneries are filthy and stinky places. You better keep your breath while visiting them. They use pigeon guano to soften the leather.