Jody at Gumbo Lily wrote a wonderfully descriptive post about how her own grandchildren play near her while she works. Of course it makes most of us remember our own childhoods and the kind of fun we had all by ourselves. If you haven't already, I hope you will comment on her post or here with some of your memories along that line.
Of outdoor play, I remember in my own early years making dolls' houses in the dirt under orange trees, and the classic mud pies. If the children are "entertaining themselves," and the adults are taking the opportunity to get some work done, the vast majority of what children do with their time is undocumented, and likely unremembered also. Two more ways we don't control it.
This picture was taken just after we moved from the country to the city. When we had a huge garden next to a cow pasture and a blackberry bog, across the road from an abandoned orchard, we had no snails. So when we moved here where we now live, they were a new and fascinating object of play, which I definitely did not introduce as a science topic. I don't know what all went on with those snails, but I had to laugh at the way every little thing can be a toy on what Jody call's God's playground.
Now read her blog, because her examples are nicer.