My house was built in 1979 and it has double paned windows. That was RARE back then.
It also has a heap of insulation in the attic, but I think some of that was added after it was built. I need to come up with a powered attic vent fan, though; I saw a solar powered one which was pretty interesting. I need to build one of those insulated boxes to go over the attic door, there is a 'pop up' type available for ~$75 but I think I can beat that price.
I'm big on ceiling fans instead of A/C unless it's just too damn hot/muggy. But one good thing: it's possible to run the A/C for a short time to remove the moisture buildup, then thanks to all the insulation the house stays comfy for a good while afterwards.
But as far as retrofits: replacing drafty windows may not actually pay for themselves, but if it keeps you from feeling 'cold' even though the house is at a decent temperature then yes it could help keep the occupants from turning the heat to Sahara levels. So in that sense yes they can save a good bit of energy.
Weatherstripping doors is another big one. If you can see light around the edges, it's not properly sealed. A storm door can help a lot here as well. If you touch your outside door and it's either hotter or colder than the surrounding wall, consider a storm door.
There are solar water pre heaters out there and they can also be pretty easily built. You put them in the water heater supply line, even if the water coming in is only, say, 85 degrees then that's still less work for the water heater.
