Have a go at testing materials and shapes. Make your brain think about new ways of testing and not giving up if it doesn't work the first time.
See if you can make your aeroplane with 'cargo' on it - does the shape make a difference or is it just the materials?
Have a go at making a marble maze, there are some ideas above but you can be as creative as you want to be. Once you have made one keep practising and encourage your brain to focus and work harder