Fresh eyes on the problem is always good, usually after lunch or you have slept on it.

I find when my brain starts to grind to a halt I just go an do another un-related task to force myself to completely forget about the original task. It does help because when I go back to the original task my brain has to context-switch back into it and in doing so it may look at the problem a different way.

Extra holidays would be great! However, I think more holidays would make some engineers more productive and some less productive... I'm not sure if Google is still doing their 20% time, and if they are I know it doesn't apply to all the FTE engineers.