* zwave motion sensors that can turn on the stairs lights at night when I start to walk down
* door/window sensors that will automatically turn the AC off if they are open for more than a minute.
* several methods of presence detection for anyone in the house, and ensuring the doors are locked, lights out, and garage doors down if nobody is home (after an announcement on the google homes just in case anyone somehow is still home)
* automatic notifications to my wife's phone when i'm on my way home after work so she would know when to start dinner (we don't use this one any more as I work from home, but it beat getting phone calls and me guessing how long I was from home every day!)
* notifications that show an image of who is at the front door when there is motion there
* an alert if a door in the house is left open for more than 1 minute
* a vibration sensor combined with a tiny hall sensor on the door of the washer/dryer to notify me when laundry is done (trying to measure power usage on 220v wiring was looking to be more annoyance than it was worth, so i went a different route!)
And one of my favorites that i'm working on right now:
* a system to measure rainfall amount and only turn on the reclaimed water sprinklers when it's not enough, but adjust the runtime accordingly.