Many consumer temperature, humidity, rain, and wind sensors use simple radio protocols in the 433 MHz band to report their readings back to their indoor display unit.

Stick an RTL-SDR dongle on your computer and use the program rtl_433 to monitor that traffic. It comes with built-in decoders for the protocols for a very large number of those sensors.

I've got such a setup and I can see not only my one sensors (An AcuRite outdoor thermometer, and the two sensors from an AcuRite fridge/freeze thermometer) but also a few neighbor's temperature sensors, humidity sensors, soil moisture sensors, wind speed and direction sensors, rain gauges, and their tire pressure.

You can set up MQTT and have rtl_433 report the data that way, and you can have Home Assistant get that and display it. It knows how to handle a variety of sensor types.

I haven't done that yet. I just have rtl_433 log everything by appending JSON records to a file. A script reads that, finds the sensors I care about, and stores their data in an sqlite database.