balloon powered lego car...that looks really cool!...have a contest...see whose car goes the farthest!
Also on these boards
Use Legos to build race lanes for cars - the post shows how to build a simple mechanism to release all the cars at once! Totally fun!
DIY Portable LEGO Kit with Free Printable Activity Cards. A great idea for those times where you have to wait (Doctor's office, restaurant) or when you are traveling (great in the car or on a plane)! From Fun at Home with Kids
Repurpose a large pump dispenser to fill water balloons.