Paper Airplanes never get old. So many different types, you can color them to look like fighters or be silly. And it's always fun to see which can go the farthest, stay in the air the longest, or do loops.
Paper airplane links:
Something to aspire to... or it's fun just make a bridge or a fort.
How long can you make it?
How much can it hold?
Toothpick engineering websites:
Being an engineering major I like to build. Here are a few fun sites. Bridges, DIY Newton's cradle and the PBS Design Squad link has lots of suggestions.
Craft building links: