Blow the dust off the ULP and put it to work for a change, bit-banging some APA-type RGB LEDs. I wrote a little C++ library to do just that, built with Arduino in mind (but it will work with esp-idf too). It can also be used as a controller for FastLED (and NeoPixelBus, kind of).
https://github.com/boarchuz/ULPAPA
There isn't really any practical use for it, except maybe very fringe cases where RMT/I2S peripherals are busy or conflict with sleep behaviour.