Easy answer... cyberstalk them and show off how much you know about their “private” lives.
Sure I drink tea, if someone asks me I'm not going to lie about that.
Worse, you can't predict what will happen to society and its rules, up to the point of "normal" stuff becoming illegal. Take as an example the (new) laws on pornography in the UK. What if have an under-18s minor in your house and your trashbags are full of pornmags?
My impulsive response is that any data from which identity can be inferred is PII.
I'm not smart enough to understand the maths of differential privacy. I gather that it's a calculus for determining how much to fuzz the data (points) to anonymize. So car rentals may need to decrease the accuracy of both timestamps and locations to create hash collisions with other rentals.
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Maybe there's a way to create logical "data diodes", for lack of better term, for the notion of data flows and relationships go only one way. So for electricity, each meter has GUID, which is never referenced directly, meaning any external reference (foreign key) references an opaque identifier which can then be dereferenced within the system. Then anything referencing the meter will be issued its own opaque identifier, so that no two references can be linked from outside the system.
I'll have to dig out my copy of Translucent Databases to see if I'm making this up or repeating something that author had already thought through.
All that happens is scary privacy themed movies get made.
Someone needs to reinvent services with privacy in mind and wait for the next generation.