There's some sort of metaphor in this, Tetris set in so huge a playing field that it becomes pointless to play. (Via Kottke.) Formal limitations are generative; unbounded free play, not so much.
This game, for instance, (via Joshuah Bearman) is strangely compelling. It consists entirely of trying to guess geometric centers and align parallel lines. It's like a training program for membership in some elite band of speculative surveyors. It has built my confidence in hanging pictures correctly, so that they are not crooked. I think eventually I'll be able to throw away my level altogether.