The resonators on cheap Pro Mini clones have so much thermal variation, that you can base a temperature sensing method on that alone: 

https://thecavepearlproject.org/2018/02/03/measuring-temperature-with-two-clocks/

There is some rising/falling hysteresis that I'm still chewing on, and when I get brave enough to mess around with fuses, I'll re-try the method with the internal oscillator to see how that works.