I quite like the idea of the story starting off in a regular, boring universe, and then stepping through a crack, or tear, in space-time into a parallel universe where all the quantum strangeness happens. This idea may have been pinched from xkcd. However, I think the idea of the background being ‘ripped’, with normal walls at the edge, and the ‘quantum universe’ in the middle might work.
However, I currently can’t decide whether this should be the way into the story, or whether the user should get ‘shrunk’. If this were the start, the ripped world wouldn’t be relevant (unless the ‘rips’ in spacetime are only visible to the very small person), so I need to think of a good image for the quantum universe.
There are a lot of representations of parallel universes as ‘bubble universes’, with multiple universes ‘bubbling off’ the old ones, so perhaps I could use this motif.
Since it’s also about spacetime, perhaps the stereotypical ‘grid-curvature’ idea could work into this somehow – white bubbles on black backgrounds?