Feynman: Surely You're Joking, Mr. Feynman! - great inspiration for regaining playfulness. Particularly this quote applicable to many burnt out developers I believe:

Physics disgusts me a little bit now, but I used to enjoy doing physics. Why did I enjoy it? I used to play with it. ... So I got this new attitude. Now that I am burned out and I'll never accomplish anything,...I'm going to play with physics whenever I want to, without worrying about any importance whatsoever.