You will need a box of white cake mix, plus the ingredients called for on the back of the box. It does not matter whether your cake mix is 15.25 ounces or 14.25 ounces - both will bake up well in a 13x9 dish.
Cool whip helps this recipe stick to the quick and easy motto. I wouldn't suggest regular whipped cream on top, as it will deflate after sitting out for a bit as the air bubbles relax after whipping. However, you can always make some homemade stabilized whipped cream instead.