The coolest thing I ever found was some federal property where the guards went home on the weekends. This was back in the 80's and the local sheriff would drive by and just watch. It was out of his jurisdiction. I don't think there's any unattended property anywhere these days.
Anyway, all kinds of things went on there and nothing really bad ever happened that I know of. Parents took their teenagers over to practice drive, we raced karts and nearby they flew remote control model planes. When I talk about the good ol' days, I mean it.
This is the place in Laguna Nigel, CA.
http://maps.google.com/maps?ie=UTF8&ll=33.560779,-117.711307&spn=0.005972,0.012939&t=h&z=17
