Ecowitt.com or Amazon (their site uses Amz FBA, btw)

Temp, humidity, pool temp, soil moisture  --or up to complete weather station.

Smarthome NOT required for a simple network, one 'gateway' (hub) generates its own web page that -once setup is complete- is accessible via browser using LAN IP.

Cheap-ish, as sensors go.  RC rates this one 10/10.

If you want to try it, here's the starter kit (US version = 915 MHz)
ECOWITT GW1104 Wi-Fi Thermometer and Hygrometer Sensor, Including GW1100 Wi-Fi Gateway and WH31 Wireless Multi-Channel Temperature and Humidity Sensor, Dip Switches to Switch ℃/℉, 915 MHz
https://www.amazon.com/dp/B081P63GFB

Rupert, the heavyweights at Hubitat have integrated Ecowitt as a sub-network. You can use the Ecowitt sensors as devices.