Skip Casablanca. Skip Tangier.
Maybe add a quick trip over the High Atlas mountains to Ait Benhaddou (though you will probably see a little snow on the ground going over the mountain pass - you can stop and have a snowball fight in Africa!). Maybe add Essaouira. Maybe consider short side trips to Meknes, and Volubilis.
Don't waste any time on anything between Marrakech and the coast, and don't bother with the north coast itself. The good stuff is all further inland (except for Essaouira, which is lovely, but a bit of an outlier so maybe not doable if you are short on time). With 10-ish days, you'll have enough time for all the above if you move fast and are efficient.
Consider an efficient route like flying in to Marrakech, and flying out from Fes. Flights are dirt cheap, and you only waste time and money getting to/from the coast and slow ferries. Above all, prioritize Marrakech and Fes, don't miss either (but neither needs more than 2-3 days each).
Prepare to fend off a lot of would-be "guides" and other touts wanting to "make friends" and "practice their English." The touts are not as bad as they used to be (government has cracked down a bit on them to leave tourists alone) but as a single (presumably male) traveler, you will be a magnet for attention. Just get used to saying no and don't feel bad about ignoring them completely. They're just an annoyance and won't hurt you.
Despite what many will tell you, you do NOT need any sort of "guide" in the medinas - they're just very old neighborhoods in very old cities. Yes, the streets are maze-like but nothing bad will happen to you if you just wander around in there, eventually you will recognize things you saw on your way in, and you can easily find your way out if you have any sense of direction at all (I find that a tiny compass can come in handy for places like the medinas). No need to fear or fret over feeling a bit "lost" in there initially.
Initially you may experience some serious culture shock, but it should quickly pass as you start to notice all the European tourists, football hooligans and stag parties, all the familiar trappings of more familiar places (ATMs, 7-11s, familiar chain shops and products). Yes, it's exotic (it is in Africa and it's clearly an Islamic country). If you can speak even a little bad French it will go a long way (plenty of locals speak a little bad French, too). But Morocco is full of European tourists and is easier to do than you think it will be. It's more like Cancun than Kabul: a cheap fun-in-the-sun mass tourism destination for western Europeans. You will do fine.
Have fun and الله معك
