Filed in things I foolishly never realised: a propane tank is shaped like a lego head. WHOA. So all you have to do is spray paint your propane tank yellow and draw a face on it and you'll have a Lego Head Propane Tank!
The full instructions are at Instructables but it's just as easy as it sounds: a lot of spray paint and enamel hobby paint for the face. Congratulations, you just made the best conversation starter at a BBQ ever. [Instructables]