I agree with AvidLearner. I also believe that the definition of a successful science fair project may be determined by the motivation level of the student. That is, some students hope to take their project all the way to the regional or national science fairs, while others are content to have some fun and complete their project in time for the school science fair. Whatever your level of motivation, there are many sites available to help define an idea and thoughtfully complete the steps necessary to put together a high-quality experiment.
It depends on your perspective. From the school's/teachers' perspective, a good project is one that follows the rules and guidelines they give for science projects. From your child's perspective, it's one that is fun to do. From your perspective, it may be one that's easy to do, and doesn't require a lot of your time and $$$.
Look through my list of sites with science project ideas. Cool-Science-Projects, Science Buddies and Science Fair Idea Exchange are just a few of the sites with good projects: