Not all survivors of the Crap Times did so by going into seclusion. The Halflings are an energetic race of fun-loving post-humans who spent most of the Crap Times out in the open, in the ruins of the cities the bombs had struck. Some may have sheltered the initial blast among the bunkers of the Gnomes, while others gradually crept down from the hills after the dust had settled a few decades in. When the winds change and the fallout returned, they buried themselves in the ruins of their ancestors to weather the storm. They are a survivable folk, able to find food and shelter in almost any disaster, and most are even able to eke out joy among the worst of times.
Because they spent their years among ancient human structures, Halfling culture is riddled with legends of the past. Some are accurate (or mostly accurate) stories passed generation to generation, recounting the events of civilization, including stories of the time before the Big Wake Up. Even the names of these events are taken from the Halfling vernacular, demonstrating their simple nature and childlike amusement.
Halflings like food and pleasure and freedom, and dislike anyone telling them what they can't do. They view the world with wonder and amazement, but also as a place they have dominion over. This isn't driven from a sense of hubris, for more than any other post-human, the Halflings are very aware of their own fragility, but rather a place of freedom. They tend to see the world, and magic and technology, as barriers to be overcome on their way to finding permanent joy.
Halflings are divided into two sub-races, though these subraces are less like true races and more just minor adaptations to different altitudes.
Lightfoots
Lightfoot Halflings live in the sky, swinging from ropes and dodging death before breakfast -- or at least second breakfast. The Lightfoot Halflings make their homes in the tops of the the skyscrapers, finding relative safety from the various creatures that wander the wastes by dint of just being hard to reach. They consider the occasional falling death a small price to pay for the carefree life their homes provide for them. Lightfoot Halflings are jovial and worry-less, enthusiastic, and afraid of almost nothing.
Stouts
Stout Halflings live on the ground, where they are safe from the tumbles that sends their occasional brother's corpse to their domain. But other dangers await them there, including predators as well as other survival threats like tainted foods and waters. Because of this, Stout Halflings are hardier than their airborne siblings, and more capable in a fight. They tend to be more cautious, and know when is a good time to flee for their own safety.