Oddly, given its founders, very much the opposite of 'weird spiritual stuff'. It was set up by Theosophists, and for the first 70-ish years run by Quakers, because they objected to the heavy pushing of CofE Christianity in the schools of the time. So it was very firmly secular. No hymns, no prayers, nobody - pupil or teacher - allowed to preach or do anything that could be seen as pushing their religious view onto someone else, and no RE lessons (in my day we did a general World Studies class - which included learning about all sorts of religions but also wider aspects of culture). The only exception was the food (vegetarian, because of the Theosophists). And the general way it ran had a very strong Quaker influence - first names and no titles for all, lots of committees, decision by consensus, and involving everyone in making those decisions. Both junior and senior schools had a school council with elected representatives from each year and a set of elected counsellors from the top year, who made decisions on everything from lunch menus to discipline questions (general principles and individual cases).
The council is still going, last l heard, but I'm not sure of the details these days - whether it has retained as much power and autonomy as it used to have.
Minimal rules, but it wasn't a Summerhill style free for all. No meat, no religion, and general respect for others pretty much covered it (switching to a school that had 2 sides of A4 just for uniform rules was quite a shock!).
Early on there was the choice of opting out of lessons provided you didn't disrupt anyone else, but that is long gone. Up to age 7 used to be Montessori system, with a lot of self-directed learning carrying on up the school. Once a term (I think, may have been once a year) there was staff holiday, when the 6th form took over teaching and running the whole school.
(Speaking of holidays - check how term dates work. Because of large numbers of international pupils they used to have very long xmas/Easter/summer breaks, but only 1 day for half terms and never took May bank holiday.)
It was very much about growing the 'whole child' - before that was really a concept in the wider world. And self sufficiency in all senses - from the independent study to growing much of its own food. And fresh air - the Edwardian belief in verandahs and kicking children outdoors at every opportunity informed a lot of the architecture well into the second half of the 20th century.
A quite high proportion of places (think it was about 20%) were reserved for children the state system at the time couldn't or didn't want to cope with for assorted reasons (bear in mind that I'm old enough for 'special schools' to have been the default for children with physical disabilities as well as SEN). Music, drama, art etc were valued as much as English and maths, asking questions and finding your own path were encouraged, so it was the sort of school that produced jazz musicans and film makers rather than members of the Cabinet.
AA Gill went there, and spent much of the rest of his life mentioning in columns how much he hated it. My class turned out an astrophysicist, a Cirque du Soleil trapeze artist, a human rights lawyer, an accountant, a couple of editors, an artist, a musician in a group you've probably heard of if you're into folk music and another you won't have done unless you frequent a particular cafe where he has a regular slot, a drugs mule, and a school science technician (plus a few I've lost track of). Most of us ended up somewhere we wouldn’t have expected (one of the editors is dyslexic, the trapeze artist hated sports lessons, the accountant failed maths), and a lot took circuitous and unconventional routes to their destination. But most of us are happy - although often with a lingering bafflement at the way the wider world works.
How much of that still applies now? The council structure, I think; vegetarian lunches last I heard - but not meals for boarders; a good amount of outside space; and plenty of international links and exchange opportunities. Not sure how much else.