I was hanging out with my four-year-old earlier today, and we were (just bear with me here) opening imaginary un-birthday presents for Oswald's birthday. All of the presents that he received were games, which we would then open and play.
Mind you, these were imaginary games, so they were invisible.
The first one was Obstacles, which is actually a real game that we play fairly regularly. For the imaginary version, he would make up an obstacle, and then we each explained how our pretend tools would help to overcome that obstacle. It was fun, but it stuck fairly close to how the real game actually goes.
Subsequent games were entirely made-up: Cat's Crunchies, Deer Tree, Moo Doo, Mummies Muh-Gummies, etc. (the 4yo came up with the names of the games), as were the rules and gameplay.
My favorite moment came while we were playing Deer Tree. During my turn, I spun the imaginary spinner. "Okay," I said, "it landed on Red. What happens when I get a Red?"
The four-year-old's reply, without skipping a beat, was that I should start my own game and that we would then continue playing in parallel.
So, essentially, he had just forked the game.