I've been to both locations several times and personally I would not drive from Zihu to La Manzanilla, too much time spent in a car. Try driving to Barra de Potosi just south of the Zihu airport or going north to Troncones both are great little places. Take a water taxi to Playa Las Gatas. Check out http://www.zihuatanejo.net/ if you haven't already. Make a different trip to La Manzanilla. There are so many great places to see nearby this town as well.