A Model of Play
A Model of Play is a fascinating poster (available as a PDF or as images) that takes what seems like a very game-grammar point of view on the concept of play — even freeform play.
In play, one of the primary goals is to have fun — to continue engaging in the conversation that creates fun. Individuals choose the means for achieving that goal; they choose the topic of conversation, for example, which game to play. Within a topic, they choose different strategies and pursue a series of sub-goals, adjusting means according to their effectiveness. Goals and sub-goals and associated means form a tree (or web) of possibilities for action.
Among the grammar principles that are mentioned is the notion that play always requires two, even if the second person is a “virtual person.” The notion that interactivity is inherently a conversation can be traced back to at least Chris Crawford, of course.
Also cool is the “step by step” logic version found here, which builds the poster argument by argument.

Those of you who have read 