You might gain a bit more awareness from the act of sitting if the object that's supporting you sinks slightly beneath your weight. The Jerry Bench has been designed to do this with the use of a metal coil as its base.
The collaboration between Shaul Cohen and Oded Marcus has produced a sort of of rocking chair for an outdoor setting. On top of its tough spring is a broadening high-gloss box that establishes the object as a functional stool. With the helical bottom rooted into the soil, the sitter is permitted to sway and influence the angle of the block for his personal comfort.
Each Jerry Bench can accommodate only one person, but the minimalist lawn chair looks quite good and would prove to be useful if multiple are lined up together in a row.