Bzupke and his son made this amazing steerable blimp using the LEGO RCX. via NXT Step
Using an RCX, 3 Motors, the Infrared Remote, Large trash bags, and lots of helium, my son and I created a steerable blimp. Two of the motors controlled propellers (made from a LEGO propeller with larger blades made out of packaging plastic). The third motor controlled the tilt of the two propellers. Each propeller can be separately controlled. For example, to rotate the craft, one propeller would be driven forward and the other backward.