One option I have considered is the old family summer house in the Stockholm archipelago, more specifically located on
Ljuster?. It is now owned by my uncle, so permission from him would have to be granted in order to use it.
If going by car, it is accessible by ferry. There should also be boats going from Stockholm. Not entirely sure how many people could be accomodated, but a bunch at least. Setting up a tent in the yard is probably not a problem, as I'm sure that's been done before. No running water and only an outhouse for when such business needs to be attended to. Two beeches within walking distance. No store within walking distance, but should be one within driving distance.
I haven't been there for 15-20 years or so, so my information may not be entirely reliable. Also open for other options.