I like the idea of building wind-powered cars, either pushed by a fan or students blowing on them or maybe a balloon. This would also incorporate well with a lesson on force and motion.
Homemade Propeller Car - Great first electronics project for kids. Includes full step-by-step picture tutorial plus a video of car in action.
How to make these 10 cardboard roll crafts. A Thrifty Mum: 10 Cardboard roll crafts including a rocket that flies and many more (loo rolls, cardboard tube or toilet rolls)