A magical backpack of this sort appears quite ordinary—well-made and well-used. It is of finely tanned leather, and the straps have brass hardware and buckles.

There are two side pouches, each of which appears large enough to hold about a quart of material, but each is similar to a bag of holding and will actually contain material equal to as much as 2 cubic feet in volume or 20 pounds in weight. 

The large central portion of the pack can contain up to 8 cubic feet or 80 pounds of material—and the pack has an even greater dweomer:

When the wearer reaches into one of its parts for a specific item, that item will be on top of whatever else might be in the same part of the pack. Thus, no digging around and fumbling is ever necessary to find what the haversack keeps safe.

Because of its dweomer, Heward's Handy Haversack and whatever it contains gain +2 on all saving throws for any reason.