Fes is a beautiful mix of middle ages and the modern world. The extraordinary medina city of Fes El Bali is worth a few days walking in itself. Other highlights include the Merenid tombs, the Royal Palace and the Mellah. Fes was Morocco’s capital for more than 400 years and is still considered the religious and cultural center of the country.
You could see Chefchaouen on the way down to Fes. It’s an old Mediterranean style town that lies along side a river in the Atlas Mountains. Very peaceful and tranquil.
Casablanca is almost directly west of Fes on the coast. It has the Hassan II Mosque, with strong Moorish influences, and one of the largest Mosque’s in the world. Tours are given so you can find that easily online. The famous movie was based on this location too.
Marrakech, is directly south of Casablanca. There is a main train line between these places. The night train also goes from Marrakech to Tangiers (the port where the ferries are). Marrakech, is very interesting to see and you can easily stay for several days. The markets and the history is amazing. Do some research but, definitely worth the trip.
From this point if you have time I would recommend going south into the Sahara Desert. This would require much more time (like a week or more) but I would like to do it when I return there. You can hire camels and do a trek across the desert. If not, directly west of Marrakech is Essaouira. This is the oldest city in all of Africa. Lots of history and an interesting city on the Atlantic ocean. I recommend trying some Tangine cooking. Fresh fish and vegetables and many restaurants will let you bring your own food from the markets to prepare. Morocco has incredible leather goods and rugs. Bring stuff to trade. Western clothing and whatever……you will be surprised what you can barter with.
The Hassan mosque itself is the reportedly third largest mosque in the world (in terms of the built up area), after the Masjid al-Haram (Grand Mosque) of Makkah and the Al-Masjid al-Nabawi in Medina.