An exercise system consisting of an inflatable assembly and stackable weight elements that can be used to support the inflatable assembly at different elevations. The inflatable assembly presents an inflated curved surface for body support. The stackable weight elements each have annular bodies that can be selectively filled with fill material. Each of the stackable weight elements have crenelated sections that intermesh when the stackable weight elements are vertically and concentrically stacked. The intermeshing of the stackable weight elements enables the stackable weight elements to remain highly stable when stacked and used to support the half-ball assembly.