One of the most surprisingly comfortable surfaces I've ever reclined on was in a teepee-like Sami structure in Sweden's Arctic Circle. The "floor" was thickly layered with birch branches covered in reindeer hides. The dense collection of branches provided a surprising amount of spring and bounce; it was like lounging on an enormous circular bed, and opened my eyes to the fact that you needn't sleep on a mattress to be comfortable.
Speaking of which, Gothenburg-based Wranne Fahraeus Design (working in conjunction with Chalmers University) has designed a mattress-free bed made of upholstered sprung steel, shown above. As WFD explains, "The geometry of the leaf springs and the extremely high yield point of the selected steel provide the desired elastic properties. The design can also be used for sofas, armchairs, etc., with comfort equivalent to that of a contemporary mattress." The concept is that the "bed" would ship in a small and reportedly lightweight package, and the user could then assemble the steel leaf springs into the final shape.
The design was selected as a finalist in the Swedish Steel Prize, an international design competition seeking innovative uses of the material. (You can check out other finalists here.)