I presume you know the story of Feynman and spinning plates [1]. Sense of play is important for productivity, sometimes it is more than just important, it becomes a necessity

[1] http://www.physics.ohio-state.edu/~kilcup/262/feynman.html

    The diagrams and the whole business that I got the Nobel
    Prize for came from that piddling around with the wobbling
    plate. -- Richard Feynman