We live in amazing times. For $2.50 each I got some ESP8266 on AliExpress and flashed nodemcu on them with minimal fuss. I've now got kilobytes of flash spare on a WiFi device with 10 digital IO, 1 analog input and a standby current draw of 78 micro amps!
Thanks to the author of this for the instructions. Basically just put the FTDI adapter in 3.3v mode and get breadboarding with the magic pinouts described on the esptool github page and flashed the latest firmware binary blob. Reboot and I get a Lua prompt via minicom. Worked flawlessly.
Next up, DS18B20 temperature sensor, an I2C humidity sensor and some soldering. :)