This. Luckily the gubmint did it for me for free, but it's made a TREMENDOUS
difference. Since I live near an airport they did it to my whole neighborhood for free, replaced all my old windows with new triple pane windows and doors, and they sprayed insulation everywhere. My heating/cooling costs are roughly half what they were before, and you could cut up a dead hooker with a chainsaw inside and no one in the front yard could hear it.
My house in Memphis I put the solar tints on the skylight windows, and that made a good difference as well.
Aside from savings, I just redid my kitchen, and while expensive, I love it. Having it set up how you like it is invaluable. Everything is where it should be, and it's all stuff I picked out. Then again, I went from a 1952 pink tiled kitchen, with a pink stove and even the insides of the sink were pink. Now I have a fully modern one. I got it appraised for a refi, and the value of the house went up about 90% of what I spent, which isn't bad.
If I had extra cash now, I'd love to get those remote controlled vents that you can turn on in different rooms to open/shut/ and even pull air into a room.