This paper describes the design of a prototype of an expendable two-wheeled rover called PUFFER that is meant to explore the difficult-to-reach regions of Mars. The mission plan is to stack multiple PUFFERs in a cavity inside a parent rover like Curiosity. When this parent rover encounters a natural feature of interest that is inaccessible to large rovers, it can eject a PUFFER to explore further. PUFFER stands for "pop-up flat-folding explorer robot," so named because it can fold itself to fit into tight spaces. PUFFER's chassis consists of a 3-D linkage that can vary the sprawl angle of PUFFER's wheels. This ability to sprawl, besides letting multiple PUFFERs fit into a parent rover, improves PUFFER's ability to climb steep slopes. Each wheel, furthermore, is outfitted with nitinol brushes that are optimized to maximize ground traction. As a result, PUFFER is able to climb 45 degree rock inclines that have surface roughness on the order of 1cm (which is almost half of PUFFER's folded height). PUFFER also has a collapsible tail, is able to flip itself over, and is designed to survive large drop heights.