We just replaced our hot water heater (gas). Our plumber recommended that we did not get an on demand one, as our basement is really the lower level of the house, where we have a family room, bedroom, and bath, in addition to the utility room. I can't remember the reason, but they have a propensity toward leaking more than regular ones and we didn't want to take the chance of that happening and the water leaking down to the family room. We bought a regular, energy efficient gas water heater.
We have totally remodeled a 22 year old contemporary home that had never been taken care of. The heating and AC was in unbelievably bad shape. Some rooms would be burning/freezing, while others got nothing at all. Our AC/heating guy couldn't figure it out. My husband went in and found that someone had done all of this weird stuff to the vents... unbelievable. He fixed it. The biggest thing we have done to reduce energy costs is last year we put in all new windows. This was a huge expense. I can't tell you how many windows we have in a 4 level home. But, our heating/AC bills are half of our neighbor's and our houses are the same size. Even on really hot days, I don't put the central AC on until noon (unless it is really humid).
Ditto for the Toto toilets. We have 2 and 2 others that are also energy efficient, but cheaper (from Home Depot).
We had a whole house fan in our last house. I liked it for the days when heat would build up in the house, but it wasn't really hot outside. Putting the fan on for a few hours would cool the house down for night time. It was noisy, though. And, not a replacement for AC, which I couldn't live without anymore than heat.