Then number of possible plant growth experiments one can do with a clinostat or centrifuge is virtually unlimited. Be creative, but also be a scientist. In essence, a scientist is a person who thinks of a hypothesis and then develops an experiment to test that hypothesis. Of course, some questions are better than others. Growing plants on a centrifuge which increases the gravity they feel will never tell us why the sky is blue, but it might help us learn more about what makes grass grow.
All good hypotheses start with a good question.
After the question, the next step, and this is the difficult part, is to rephrase the question as a statement which can be proven false by an experiment. This implies what kind of experiment should be done. In the first, carbon dioxide and light will be altered. In the second, gravity will be altered. The first question could yield some very interesting results but, as before, some statements are better than others.