In Surely You Must Be Joking, Mr. Feynman, Richard Feynman tells the story of how he managed to end a dry spell where he couldn't come up with good research ideas.

He was in a cafeteria, someone slipped, and accidentally threw a plate into the air. Feynman could see it spinning, and could see that it had a wobble that spun, and wondered if he could figure out the ratio between the two.

The piece of mathematics that he worked out had no particular purpose. But having it turned out to be essential later in the work that earned him a Nobel prize.

Never underestimate the value of play!