Any of a number of things. I personally like leaving my work alone for awhile and going back to it later because writer's block (for me, at least) tends to indicate that I'm wearing myself out over this idea.
Sometimes, I also look for inspiration. Look at art, listen to music, watch something, go out for a walk, read a book -- anything that isn't writing.
Yeah, it may seem like you're not being productive if you just leave your work alone, but for me, sometimes, I just need to give my brain a break for a moment.