A classic teen hangout spot in my area was “the creek” (that is, the San Francisquito Creek, which divides Palo Alto from Menlo Park). There’s a place to access the creek bed (which, because this is California, is dry for most of the year) from the neighborhood and you go down there to hang out in the shade of the trees and smoke weed (commonly referred to as “creekin’ it”). Never got in trouble there although there was certainly some risk. Otherwise, Stanford campus was always a good place to hang out and walk around. I didn’t do a lot of loitering as a teen; we would more often be walking or driving around aimlessly.