Also anything you generally find relaxing that isn’t intended to be.
For reasons I cannot articulate, I never liked intentional ASMR videos, but give me an upload of a three hour video course about a word processor from the 80s or a training video about how to take a sonogram of the kidneys and I’m all in.
As for generally relaxing stuff, for some reason I find recordings of radio comms, like storm spotter nets, to be relaxing. Also local news coverage about major weather events.
And to you UK lemmings I know about the shipping forecast.
Maybe not strictly unintentional, but I used to love watching Poppy’s YouTube channel. Her videos, especially the older ones, have pleasant ambient music and overdubbed vocals that definitely tickle the brain. The light satire is fun too, and some of the videos start to lean into the world of fashion. Her newer stuff is darker and heavier, less ASMR-like but still interesting.