Nuclear war, zombie outbreaks, meteor strikes, economic collapse, alien invasion, no matter how it happens an apocalypse makes for a unique and engaging setting to run a campaign in, even for a jaded GM.
Here’s a few quick tips that can make a good zombie or other post apocalypse game a great one, and keep the group coming back (from the dead?) for more.
The Petty Kingdoms is a region consisting of hundreds of small states. Some are kingdoms, some are city states, some are large farming communities, some are duchesses and baronies and so on and so forth. Within a few miles the PCs might reach a new "country" with completely different rules and norms than the one they left behind. "What do you eat here? Humans ye say? Lets turn around fellas."