Not Casablanca! You may well need to fly into that city, but it's more modern/Europeanized than the other cities in Morocco and far less interesting.
I think Fez (aka Fes) was my favorite. It has a very active medina and seems to be the craft capital of the country, so lots of interesting things to see in addition to the architecture. The Wikipedia page I've linked to has a weather summary chart you can take a look at. Daytime temps may be in your range, but it looks like it will be a bit cooler than you want at night.
Marrakech gets more foreign tourists and is also very interesting. Meknes and Rabat are smaller but also attractive. With the time you have, you could see several of the cities.
But I was in Morocco a long time ago; it would be great if someone who has been there recently would comment. Be aware that--at least the last I heard--there are still lots of mostly young people (10 years and up, maybe) who want to be your guide. You will be approached repeatedly--I dare say "constantly"--when you're in touristy areas. It's not threatening at all, but it's annoying. If you're sensitive to such things, touring Morocco on your own may not be for you, and you should consider a tour, or hire a guide for at least some of your sightseeing time. If you speak French or Arabic, it will be easier to make your disinterest clear.