Violet MadderOne thing I definitely want to try, is cultivating live sphagnum moss!
It grows slowly, but it's very useful stuff even before it's broken down into peat (which takes years and years).
It likes very soft acidic water though, so the 8.2 pH & 320ppm GH stuff out of the tap here is probably not suitable.
I'm also very curious about harvesting condensation from the inside surfaces of a greenhouse, to use as a source of more or less distilled water.
RO systems use up a lot of salts etc and don't seem like a very sustainable long term solution. I haven't looked into it deeply though. I have next to $0 budget so that sort of thing is right out.
Sadly most of the places selling live sphagnum have no idea what species they're using, which bugs me.