My wife and I moved across the country and bought a 5 room bed and breakfast 2 years ago. Most of my automation and the devices I've created are geared towards the BnB. So lots of lights turning on and off based on time of day and whether or not we have guests. I wrote a web scraper to pull down guest data to push into Home Assistant. One of the really nice things that provides is last 4 digits of phone numbers get programmed into the front door lock and deleted when they check out. That code is here and is not terribly well organized https://github.com/chunkystyles/reservationsScraper
I created a salt tank level sensor for my water softener using a pair of Arduinos communicating over 400mhz RF. If I were redoing this one, I'd just do an ESP device. I also built a doorbell sensor that literally just has a photo resistor glued to the LED on one of the doorbell receivers. The code for both of those is here https://github.com/chunkystyles/arduinoSketches
I created touch screen controllers for mini-splits in guest rooms twice. The first version used M5 Stack Core 2 devices that was OK. The tiny screen wasn't great. And it was programmed using M5's visual block programming and it was a pain.
The second version is using a 3.5 inch screen and works way better. That code is here https://github.com/chunkystyles/makerfabs_hvac_remote
I have a project that I need to get started on. We have a small ice maker in our lobby and I need a device to monitor the door being left open, and whether the scoop was put back in its holder. The first part is self explanatory. The second part is because the ice refills from the top, and if someone leaves the scoop in the ice maker, it will get covered up by fresh ice. For that, I'm probably going to reuse one of the M5 Stacks and do a magnetic door sensor, and for the scoop holder a small limit switch that will trigger when the handle is in its normal place.
The BnB thing certainly is interesting. I like it, but it's not what I expected.
The radios I'm using are these https://www.amazon.com/gp/product/B09KY28VH8
I'm fairly sure the driver I'm using is this https://github.com/PaulStoffregen/RadioHead/tree/master It's been a while since I worked on these.
So one device has an ultrasonic distance sensor and a radio transmitter. It just takes a reading and transmits is once an hour.
The other device monitors the doorbell and has the radio receiver on it. Both of those things are sent to the serial output and monitored by Node-RED.
https://imgur.com/a/SQdc95d
The transmitter and receiver aren't terribly far apart. They're probably like 30 feet or so, but in the basement, with walls in between them. I didn't do any testing on how far apart they worked, but 433mhz is a pretty sturdy frequency and these have been rock solid. With the driver, they're actually super easy to use, too.