Hi mate, i'd say a week in Marrakesh is too much, it is very full on, great place for a couple of days, but not for a whole week.
There a number of trekking and now MTB agencies based there though. It only takes 2-3 hours to get up into the High Atlas. You have a Mt Toubkal to trek which is highest peak in N.Africa, there are some fanstastic waterfalls at Cascade D'Ouzoud and for scenery head to Dades and Todra Gorges, great places to see on a bike and stunning scenery.
In the other direction, you can travel 2 hours to get to the edge of the Sahara where there a number of high dunes right on the edge that you can climb that will give you a idea of it's beauty, the best to climb is Erg Chebbi.
Finally I would second going to Essaouira on the coast, it is very relaxed and a great place for windsurfing, kite surfing etc.