Actually, a tree house in the woods would be a veritable sanctuary. Far enough from the city to avoid a lot of the chance of seeing a hostile (living or undead). Also, seeing as how a zombie, to our knowledge, can't climb a tree, even those who stray would have a hard time getting to you. Depending on the forest, it would provide lots of food and fresh water, and also might be colder than the urban environment, so the zombies would move slower, and those hostile humans who are not properly prepared might be deterred.
I disagree. Shambling hordes of undead would have a harder time making it through dense forest than we would. On the other hand, climbing a tree and swinging on vines might be fun...what kind of forest is this?
An old missile silo or a Supermax Prison would be best. The prison you'd still have to clear out the inmates but something tells me that in a ZPAW situation the guards are not hanging around so most of the inmates will still be locked up. With secured grounds you could farm and raise livestock. Most of the prisons have the area around them cleared so you would have excellent fields of fire against raiders or zombies.
The only problem is fuel for the generators. If you had time and a knowledgable crew I am fairly sure you could set up some windmills and solar panels. You don't need to power up the entire prison really.