It's so interesting to see the different answers from different areas/climates. Pool pumps! Far and few inbetween her in VA! I think that there are so many things that can be done that a home owner may feel overwhelmed and just not get anything done. It's nice to see posts like Rj's that is area specific. I think it makes it easier for a homeowner to picture themselves making a few changes that they see recommended from someone who is an agent in their own area and probably seeing a lot of houses that give them new ideas.
This is a tough question and I would say it is very case by case. If you have an older house and have plenty of equity in the home in order to be able to afford the expense of new vinyl windows that will greatly increase the green factor. A bit cheaper fix is applying extra spray in insulation in the attic of the home as well as radiant barrier. I have seen the reduction in my parents house just from doing this. When running a pool or sprinkler system consider using solar power. These systems are becoming much more affordable and especially for pool pumps it can save you a nice chunk monthly. I know that was more than three and there was not any order but that is my take.