My favourite one solved a long-standing argument between my wife and I. I like to keep my office slightly warmer than the rest of the house, and she hates wasting electricity by heating it when I’m not there. She would keep lowering the thermostat when I was not working (like on the weekends), and I’d come back on Monday and wonder why I’m freezing in front of my computer.

I solved it with Home Assistant and a smart thermostat. Now whenever my computer becomes active, it sets the thermostat to my favourite temp, and when it’s asleep (or away) for more than 15 minutes, it sets it back to the “away” temp. Lights are also synchronized with the whole thing.