No, I'm not going to cover the whole planet, just the USA, because it's so fun (and relatively easy) to jack up. I just want to take a look at some resources I might use for creating a post apocalyptic setting. Personally I like to start with a nice, medium-high detail map that shows highways and freeways, major cities, and the basic terrain. Like this:
Next, I like to think about what kind of apocalypse to bring about. I do have to admit that I have a simple fondness for nuclear destruction. So let's look at some possible nuclear targets.
I should note for simplicity sake, that when I determine what's to be nuked, I choose the top cities (10, 20, 30, or 50 depending on size of country, in this case, I'd go with the top 50) and top military bases (in the same number as cities, equally divided among army bases, naval bases, and air force facilities).
Next, let's look at some natural disaster type changes, namely Earthquakes:
Sea Level Rise:
And, lastly, Volcanic eruptions:
I should note that I'm not including ice age style glaciers in the mix, mostly so I can have more desert-like wastelands.