I bought a bunch of wireless temperature/humidity sensors from AliExpress - they broadcast their readings once a minute via a small 433Mhz radio-transmitter. C

I bought a bunch of wireless temperature/humidity sensors from AliExpress - they broadcast their readings once a minute via a small 433Mhz radio-transmitter. Completely standalone, no WiFi, or other concerns. And because they're battery powered I think they're safe enough in the sauna! From there I can get the readings via a USB-attached SDR dongle, and also sniff the radio-transmissions via a WiFi connected ESP8266 device with a radio-receiver.