This has been a favorite game of mine since I was a kid... "What would I do if I was King?"
Okay, here goes...
Step One: Recognize the unalterable fact that our population needs to come down, and come down fast. Since I don't advocate the engineering of superbugs with a 95% kill rate, I would implement a series of initiatives to cut the birthrate from the current 2.1 (In the US) down to .5 per couple, as quickly as this could humanely be done. This means no child tax credits, of course, and a sterility program that pays something like $5000 for any citizen who wants to be sterilized. This would be huge burden at first, but it would probably cut the birthrate by half in 5 years or less.
Another thing I would do is to dismantle the public school system, with the exception of higher education such as universities and community / trade colleges. This would have to be done with at least 5 years' warning (so people desiring to have kids now will know there will be no public schools for them), and each grade would be phased out as the last of the publicly-schooled children pass through the system. This would free up enormous resources to cover other needed programs, such as massive public works projects dedicated to changing over our infrastructure to a fossil fuel-free society. There would be schools, of course, but it would be up to parents to be responsible for their children's education (jeeze, what a neat concept!)
Step Two: Now that we have the birthrate on a steep decline, we still have the extended-life problem to be concerned with. I don't know the particulars, but my brother-in-law is fond of telling me that something like 50% of all Medicare expenses go towards taking care of people in their last month of life. This has to end, and soon, as we need that money for more vital uses. I don't know how I would carry something like this out, but I would make certain that the amount of GDP spent for medical purposes be cut in half, and held there in sync with a declining GDP, however long the depression lasts. This would shift the end-of-life paradigm to the home, which is as it should be...what's the point of living an extra 10 days if you're going to be in a hospital room with tubes going in and out all over your body?? The sooner we end this madness, the better off we're going to be. Also, late-life procedures such as heart surgery would be rationed out by age...40 years old, yes, 75, no, that sort of thing.
Step Three: Complete changeover to a hydrocarbon-free society, to take place over 50 years. Nuclear would have to be a big part of this, as this is the only way to keep the electrical grid operating, and to electrify the railroads, etc, but I would implement massive solar power projects, build tidal generators, windmills wherever possible. This would require that any sort of obstacle to construction of these new energy sources (especially nukes) be removed, including environmental regulations, local opposition, etc. No lengthly permitting process here...just get 'em built, and fast. If France can run their country on nukes, so can we, and this addresses the global warming issue as well.
As far as cars go, their private use needs to be phased out as quickly as possible. Good first steps would be to increase the driving age to 21, mandate strict training and testing requirements (difficult enough to prevent about half the people from ever passing). Huge taxes would be placed on all new gasoline cars sold...with the lowest MPG cars having the highest taxes. Electric cars would be untaxed - if they are below a certain weight level...we do not need a whole fleet of battery-powered SUV's...LOL. But for the most part, there would be a massive shift over to mass transit, railroads, etc.
Step Four: Total revamping of tax policy...to help cut costs and centralize government functions. Get rid of income tax...tax consumption instead. Also, imports should be taxed at a high enough level to force the equalization of imports to exports...if we're running a trade deficit, it needs to be corrected by increases in excise taxes until imports drop to that or below the level of exports. This would get the jobs back home where we need them, as well as keeping our precious dollars home too. Some sort of wealth tax should be considered for corporations and billionaires, as well, bit this should be done in conjunction with other countries to prevent capital flight. (This could be done as part of a currency stabilization scheme). On the local level, I would eliminate property taxes and only allow the states to collect sales taxes to pay for *essential* services such as police and fire protection. This will allow millions of Americans to "shelter in place" with very little income, which is critical to prevent massive homelessness in the coming depression. Sure, we would have trashy parks and potholes all over the place, but we have that now anyhow, so I don't think there would be much of a difference...hehe
I could go on, but you get the idea. Ahhh, to be king, even for a day....