I used to do this when I lived in New York state and would occasionally travel down to New York city. It was stupidly cheaper to drive (and faster - which WTF whhhhhy). So I would rent a cheap spot in a garage near the outskirts of the city for the day and use public transport for the rest of the day. I remember being mad that it was cheaper and faster to drive and pay to store my car then it would have been to take the train. That's a problem. Especially when I had a train station in biking distance to my apartment at the time.

I mean, if you search around you can probably find someone willing to rent out driveway or garage space for cheap. Or else if you head to the outskirts of your city or near the industrial areas, you'll find car/rv storage lots - usually near or part of storage units. So the solution already exists.

I think it'll be a hard sell to get people to, say, approve government subsidies for parking garages to make it cheaper for people to store cars that they arent even using. Especially if street parking is already free

They do that in my town. Finding parking and garages and everything is not even an issue. I feel like you're more in a much more Cityfied area than I live in. Well I do live in a major match politen area I was making the comment the other day that the majority of the city I live in is more valley and suburbs than actual city. However at all of the train stops throughout the valley there is free parking. And you could even leave your car there for a few days if you wanted to. And at any of the major bus hub areas there's also large lots and free parking. They even provide random park and ride lots throughout the outer suburb areas so you are encouraged to carpool.