I wouldn't spend more thhan a night in Casablanca. It's just a big dirty city with slums all around it. A walk through the slums is interesting and down to the mosque. The mosque is very impressive but kind of bizarre when you figure it cost $800,000,000 to build and right across the road you have some of the poorest people in the country.
We hired a car from Casa and drove down the coast staying in fishing village after fishing village for a few nights, then came back inland towards Marrakesh and up to the Atlas mountains for a night. I would definitely reccomend Marrakesh for a few days. Loads to do there. I'd only reccomend driving there if you love chaos though