Brando
The Foldable Chair Backpack is, well, exactly what it sounds like. The bag has a chair - more of a stool, really - built right into the side of the pack.
When it's being used as a normal backpack, the metal legs of the chair fold up and are hidden behind a layer of breathable padding. They're lightweight, and the whole bag only weighs a little more than three pounds when it's empty. Plus, it's waterproof.
Brando
It's available for sale from Brando, and costs $75.