This is really an attack vector. Share wifi with them (let's assume this is easy with social engineering) sign up for fresh facebook and google accounts, look for any unsavory advertisements, blackmail.
Or the reverse: a prank app where you connect to the target's wifi & then it goes around browsing unsavory parts of the web and clicking on a bunch of tasteless ads so as to poison what ads are shown to the victim.
you can do this with google search autocomplete, and it's especially effective on corporate WiFi at revealing co-workers search interests
"Why is my..." "when do we..." "how to ask..."
"How do I..." "how to tell if i got" "divorce attorneys <city>"
While this post may seem unsubstantial at best, my point is to never search for anything personal while connected to a shared wifi network - even if you're doing it from the privacy of your own device.