A rotor blade for use in a turbine of a combustion turbine engine, the rotor blade comprising: an airfoil, the airfoil including a concave pressure sidewall and a convex suction sidewall extending axially between corresponding leading and trailing edges (20, 21) and radially between the base and an outboard tip end, the rotor blade further including: at least one mid-span shroud configured to engage a corresponding mid-span shroud on at least one neighboring rotor blade during operation; the airfoil further including an inboard region between the at least one mid-span shroud and the base of the blade with an inboard direction of the airfoil toward the base; the airfoil further including an outboard region between the at least one mid-span shroud and the outward tip end of the blade with an outboard direction of the airfoil toward the outward tip end; wherein: the outboard region includes at least two cavities (61, 66) extending from the outboard tip end inboard of the airfoil toward the at least one mid-span shroud, wherein the at least two cavities (61, 66) include sidewalls (65, 465, 466) and bottom walls, each respective bottom walls of the at least two cavities (61, 66) being at different angles with respect to each other and the sidewalls (65, 465, 466); and inboard of the at least one mid-span shroud, the inboard region is substantially solid. an airfoil, the airfoil including a concave pressure sidewall and a convex suction sidewall extending axially between corresponding leading and trailing edges (20, 21) and radially between the base and an outboard tip end, the rotor blade further including: at least one mid-span shroud configured to engage a corresponding mid-span shroud on at least one neighboring rotor blade during operation; the airfoil further including an inboard region between the at least one mid-span shroud and the base of the blade with an inboard direction of the airfoil toward the base; the airfoil further including an outboard region between the at least one mid-span shroud and the outward tip end of the blade with an outboard direction of the airfoil toward the outward tip end; at least one mid-span shroud configured to engage a corresponding mid-span shroud on at least one neighboring rotor blade during operation; the airfoil further including an inboard region between the at least one mid-span shroud and the base of the blade with an inboard direction of the airfoil toward the base; the airfoil further including an outboard region between the at least one mid-span shroud and the outward tip end of the blade with an outboard direction of the airfoil toward the outward tip end; wherein: the outboard region includes at least two cavities (61, 66) extending from the outboard tip end inboard of the airfoil toward the at least one mid-span shroud, wherein the at least two cavities (61, 66) include sidewalls (65, 465, 466) and bottom walls, each respective bottom walls of the at least two cavities (61, 66) being at different angles with respect to each other and the sidewalls (65, 465, 466); and inboard of the at least one mid-span shroud, the inboard region is substantially solid. the outboard region includes at least two cavities (61, 66) extending from the outboard tip end inboard of the airfoil toward the at least one mid-span shroud, wherein the at least two cavities (61, 66) include sidewalls (65, 465, 466) and bottom walls, each respective bottom walls of the at least two cavities (61, 66) being at different angles with respect to each other and the sidewalls (65, 465, 466); and inboard of the at least one mid-span shroud, the inboard region is substantially solid.