The worst thing is idling browsing on the web. It really drains energy
Then, if you say you have free time, there is a high chance you did not look hard enough. Is there really no document you could read, noone you could talk to ? Very often, when I find myself idle, I realize there are things I could do.
If really you are idle, check how much you really need to stay at work. In some environments, you could very well take some time off, or spend less time at work, and nobody would care. 7
In some work, you need to do sometimes 14 hours working days for long period. It would be fair, and is sometimes accepted that in exchange you go back home at noon on some other days.