I've skimmed and re-read a few books this way. And even the occasional paper.
Half kidding, but I spend it working another issue/bug or writing comments/docs.
Usually I will either update my story notes or start working on the next part of my code.
I do commit, push and build frequently though. Make it work > make it right > make it fast.
Sometimes tidying up my office. I just like to make sure everything is straight, at 90 degrees or 45 or parallel to the wall. It's not some OCD thing, just a matter of style. 5S is usually used for workshops, but I think it applies very well to the office.
A. Doodling a system diagram.
B. Updating documentation of what I am doing.
C. Meditating in order to keep my mind clear.
In practice, I sometimes check hackernews.