Slow days at work

I work at this company for about 8 months now, I am usually busy, working 5-6 hours a day. I enjoy working, it helps passes time, makes work days shorter. However, this past week there is nothing to do, i just sit at my desk, looking at old files trying to learn. I sometimes go to the factory and check on the work but thats it, and this takes 1hr.

Sorry for the long rant, but my question is what do you guys usually do on slow work days to pass the time?