I remember back in the mid 1980s there was a scheme in Lincolnshire UK where you could just turn up at a house and go into their back garden to admire their plants - only gardens owned by people taking part in the scheme, obviously. Doesn't seem to exist anymore, and I guess I'm not surprised,
There are also various events where you can visit historical buildings, and some include private homes. A lot of them seem to require booking these days whereas it used to be more just turn up.
Many listed here: https://www.heritageopendays.org.uk/