When you need more protection than a polycarbonate shell or some skin, look no further than a company known for durability. Thule offers ultimate protection in a water-resistant and high density sleeve. The zipper is fully waterproof, has reinforced corners, and an elastic band inside to keep your device in place. If you're taking a MacBook out into the wild for a day at the park or camping, a Thule Sleeve would be my protection of choice.
$49 at Amazon