Casablanca is a great destination both for the city itself and for places nearby that are easily accessible by train.
On day one I'd suggest a walking tour following the tramlines from Arab League Park as far as the Marché Central for a fish lunch. Then a tour of the Mosque and a walk (if fit) to El Hank Lighthouse, the Shrine of Sidi Abderrahmane and Morocco Mall.
On day two, a train from Casa Port station to El Jadida in the morning. Do the beach, the Resistance Museum, the Portuguese City, the Old Souk. Then lunch, fish again, and a Grand Taxi from beside the coach station for 60 dirhams to Bab el Feghali in Azemmour. Explore the medina and its street art and the river. See the Jewish saint's shrine, go up on the walls from the rabbi's square, Get a train back to Casablanca.
Day three the souks. Visit the Ancienne Médina. Have a meal at La Sqala which is built into the walls. After, take a cab to the Habous, a French/Arab fusion of bazaars. Take a look at the King's Palace (from outside), visit Makhama el Pasha, the Andalusian Music museum (Dar al Ala).
DONT visit Rick's Café.