I was visiting a buddy in north Austin a few months ago and found this spot: 30.42851, -97.78158 (paste coordinates into Google Maps). It's a power relay or whatever back down a small dirt road in some trees. You'll have shade and a little cover / privacy from passing drivers and homeowners. It's probably best to use this spot just to sleep. But in my case, I killed an afternoon there waiting for my buddy and his wife to get back from work, and no one hassled me. It was quiet, peaceful, and shady.