Building dams was fantastic.
Layer of sods, layer of stones, and repeat until it was deep enough to swim in, and hope it didn't break!
Building dens.
Some of ours were as impressive as anything Iron Age man built. Thanks to no longer used dry stone walls, in the fields behind our house.
Archaeological digs, hunting for treasure. Which was deemed a success if we found some bits of broken pottery, and an old bottle.
I won't go into the founding of the G.M. Club, which had it's own clubhouse, song, library, club oath, weekly meetings, membership cards, and it's own secret language and codes, in case you get the idea that I was a control freak, and the other ten members were coerced into becoming members.
Life was so exciting. As others have said, most of it cost nowt, but was so much fun.
I do hope today's children find the world as an exciting playground as I did.