Fes is where Morocco keeps its past, and it doesn't dress it up for visitors. The city was founded in 789 by Idris I, and his son Idris II founded a second settlement across the river. The old town, Fes el-Bali, has been on the UNESCO list since 1981 and is one of the world's largest car-free urban areas. Goods still move by donkey and handcart, and a shout of balak! (watch out!) means one is right behind you.
The medina is not pretty in the Marrakech way. It is dense, steep, loud and genuinely confusing, and that is the reason to come. Three days is the right length: one for the medina on foot, one for the Roman ruins and imperial city nearby, one for the royal quarter, the Mellah and the hills. A fourth day would mostly mean getting lost again.
Day one walks Fes el-Bali from the tannery uphill to Bab Boujloud. Day two is a day trip to Meknes, Moulay Idriss and the Roman city of Volubilis. Day three covers Fes el-Jdid (New Fes), the Royal Palace gates, the Mellah and two hilltop viewpoints. Start on a Wednesday: Dar Batha is closed on Tuesdays, the Borj Nord museum on Mondays, the Meknes mausoleum on Fridays, and Friday is the prayer day, when some doors shut or shorten their hours around midday. More on Morocco, and for the south there is a Marrakech guide.