The sustainability agenda has got to be a great starting point as it is a fairly new speciality so could also bring major career benefits in the future. You could break it down into bite sized pieces as it is a big subject to take on. Initially you could focus on either the financial or educational benefits and then develop it from there. There is plenty of information available and several support networks, so you could pick up knowledge as you go on. Sustainability will soon be topical in every school, particularly with the introduction of the Carbon Reduction Commitment, so it will always be relevant. If you are interested in following this route let me know, I always have a few ideas.