I have a whole wish list of radio waves I want to inspect using my HackRF SDR.
Most recently I've been trying listen to the subway radio communications while riding. So far not much luck as I'm still learning GNU Radio and SDR in general.
The first project that got me into SDR was my interest in how KPop concert light sticks (similar to LED wrist bands at Taylor Swift concerts, except KPop light sticks use ZWave not IR painting) communicate and sync up to songs. Still haven't gotten to this project yet. Maybe once I learn more I can try again.