Easiest change would be to stop zoning cities with the fill bucket tool in paint. Rather than one massive swathe of residential, one massive swathe of commercial, it'd be far better to create regions of medium density mixed use zones, so small apartment blocks zoned in the same immediate area as commercial use, with more dedicated commercial zoning in small, 2-4 city block sized chunks within 30ish minutes walk of most of the medium density. You can still have single house single family zoning in the spaces between, and Industrial zoning should still be elsewhere (real industry, not office parks), but mixed use development helps everywhere. Even just putting a strip mall style area in the heart of a low density residential gives the locals walking options, youth employment options, and basic services in their back yard.
But back yard is the key word, you'd have to absolutely murder or ignore NIMBYism for it to happen.
More radical options would involve explosives, but this is one of those "80% of the benefits for 20% of the work" options.