Disclaimer before my answer: I'm not American, nor have I lived in the United States. But I have formally studied suburbanisation as part of my work. Therefore, my answer is derived from my second-hand readings of United States history rather than first-hand experience.
The one "small" change I would advocate would be to substitute the automobile boom of the 1920s and highway construction during the New Deal with a railway boom instead (carrying on the older railway boom of the 19th century).
It's not really small, but it's one change with a significant impact and I've explained why I think so below.
From what I know, a significant factor that drove suburbanisation in the US was the cheap availability of capital left over from the second world war which could be put to other uses, thus starting a manufacturing boom in the 1950s. Cars, concrete, household appliances, hardware - all effectively became cheaper to manufacture, distribute, sell, purchase, and replace. This was combined with a sudden boom in consumption, as young soldiers returned from the war, obtained jobs manufacturing and marketing this newly cheap stuff and used their wages to buy the same stuff in the market. Low prices meant a single household could buy a lot - a car, a washing machine, an oven, a refrigerator, furniture and so on. However, land prices in cities were too high to own enough space for such a lifestyle. This is ultimately what drove young households to suburbs - Cheap land and housing that allowed for an expanded lifestyle driven by the manufacturing boom.
However, a crucial factor was the automobile and its associated infrastructure such as freeways and highways, whose development had started taking place in the 20s and 30s, but whose effect took off in the 1950s. The combination of highway+car promised that a household in the suburbs could never be too far away from work or entertainment in the city. The overall effect of this combination had a huge impact on how households lived - not only did it promise freedom to move between suburb and city as and when the household pleased, but it also promised that a household need not bind itself to any particular location for any reason at all. You didn't need to live within walking distance of shops if you could just drive down. You didn't need to live close to a doctor or a hospital if you wanted care. If all these things could be located within a 10-15 minute drive down a motorway (as opposed to a 10-15 minute walk or bus ride), you just needed access to the motorway. What this meant was that households could spread out, more and more. Their homes could get bigger and their neighbourhoods could cut deeper into rural areas with their only limit being the capacities of their cars and their access to the highway.
Therefore, if there is one "small" change I would suggest to cut down on urban sprawl, I would suggest extending the 19th Century Rail Boom to the 1920s and have rail lines replace road laying during the New Deal. Rail creates very different effects from the automobile. Households will still need to cluster and locate themselves closer to railway stations in order to access services. Factories, shops and key centres like hospitals can be located near stations to better access manufactured goods. Suburbanisation will still take place, but it will be of a very different form, consisting of denser, tighter railway towns and the sprawl is more likely to be linear, taking place around major arteries connecting primary cities, as opposed to sprawls that go in every direction. This also allows for more centralised infrastructure in towns, reducing resource wastage and hopefully, more connected communities that participate more actively in town planning processes.