Last year it was a space theme and the boys put together a flying saucer using pie tins and battery-powered Christmas lights. They had a "cub" scout in space by using a beanie-bear which they made a space helmet and uniform for (tin foil is helpful). They also made aliens using puff balls, pipe cleaners and google eyes.
It just goes to show me that the "judges" appreciate something that looks completely home grown, not something an adult has necessarily directed them in. So, next year I'll let the boys do all the planning on the center pieces and we'll go for a three-peat.