The secret ingredient for cool kid's projects is a low-temp glue gun. It will glue almost anything instantly, including fabric (great for zany fashions or costumes), styrofoam, cardboard, and wood. It's the "instant" part that really appeals to kids, who otherwise lose interest in projects while glue dries.
The other day I set up my son and a friend with a large box of popsicle sticks from the craft store, two glue guns, some cardboard boxes and a challenge. Could they make a bridge that would span at least 10 feet?
They quickly glued up trusses on the floor. Then they glued cross-bars across the two parallel trusses.
Then they glued a cardboard "road" on the bottom. They decided they wanted to drive some remote control zip-zap cars across this awesome bridge, so they glued a cardboard railing inside.
We hoisted the bridge, which successfully spanned the library in my office. I must admit that I was surprised the popsicle stick bridge was able to support itself over that span since low-temp glue is not very strong. But it worked.
And the road was drivable.