What about a robot that cleans your room or hover shoes that you could zoom around on. Maybe a small pill that has all your vegetables hidden inside (even broccoli) and when you swallow it, it tastes like chocolate or strawberries and cream.
This website has some great inventions that kids have designed and they have been turned into real products. I really like the scooter.
So get your thinking caps on and start creating. You might like to check out some of these great books to get you started.