Oh wow, nice topic.
None of what we did was done with the specific intention to improve. It was just to play. For sure, agree with "played and played and played". Aside from all the structured leagues games, the friend gang went to the ballpark almost daily in the summer. Didn't matter how many, we would make up some version of the game that fit. I remember the outfield along the LF lines were naturally "fenced" by the woods. It was special to hit into the woods and well worth the search for the lost ball. Only so many balls at our disposal.
Aside from at the field... (when alone) the house had a brick wall beyond the driveway with just enough grass area and slope. Strike zone and specific pitch locations were defined by different shades of brick color. I pitched for hours... was usually Steve Carlton but mixed it up from time to time. (with small groups) we had a fairly long driveway and big yard. Oversize wood bat screwed and taped along with a slightly under-inflated volleyball made for a great game, pretty much stick ball on steroids. Best friend had a perfect wiffle ball field back yard with rules pertaining to hitting various parts of the house/roof. During the winter, little brother and I had a well-defined basement game with badminton racquets and a soft foam nerf ball. We developed the full array of pitches. We would usually pick from the 70's teams - Pirates, Reds, with all of their unique and quirky batting stances and swings - copied someone different with each AB. Learned to switch hit that way.
Funny thing. Dads in that era were rarely part of the equation with sports. We created all the games on our own and worked through disputes and challenges that way too. Hard working dads were completely oblivious to what we did for play on a daily basis.