I just finished up documentation of my first Open Source Hardware project https://github.com/jeena/esp8266-button
It is a hardware button which you can press and it will boot up and connect to your Wifi, after that it will publish one MQTT topic and will turn itself off.
I myself use it to toggle a state in Home Assistant, which automates my flat, so it understands that I'm either going to sleep in the evening, or just woke up in the morning. It then can automate the appropriate things accordingly.
But with really small tweaks you can automate a bazilion different things with it. It's especially interesting because it only costs about 2 EUR to build, but also the battery should be able to drive it at least for months, in theory even for years. Other buttons like that normally cost around 50 EUR if you buy them prebuild.