Probably nothing important due to copyright issues, company time + company resources means the company owns the work.
I do wonder if this is the origin of a lot of fun projects like "HN command line client" though.
It doesn't matter that you are not writing a thing.
If you're doing it on your private machine over which you have full control then how can anyone prove it was done during work hours?
I can sit bored (with a couple of hours worth tracking in the timesheet software) for a month or two, but then I usually start looking for something interesting.
You don't need to like or "couple your soul" to your job, it's part of your life and life shouldn't suck because there is a very good chance that there isn't another one.