From where I'm sitting this seems to be a really good problem to have, btw!
One of the things you could do is read the classics (or anything else that is of interest to you). Presumably, if you can check out the MMM forums, you can check out any public site on which novels are available once they are no longer copyrighted. Or any e-book you've checked out from a public library.
I only ever had one job where I was in a similar circumstance but I felt that passersby could easily see my computer screen. I didn't want to be seen not working so one thing I did was write long-hand journal entries for myself. I like to think it looked like I was hard at work. I guess you could do something like this within an email application, to send to your personal address, and it would not appear to anyone as anything other than work. Occasionally, I would do free typing tests on-line to increase my keyboarding skills.
You could also take some time to just wander around your workplace and get a modicum of activity in your day, particularly if you are in a multiple-story building, by climbing a few flights of stairs at various intervals.
That's what comes to mind with little thought... but, hey, it sounds like you have the benefit of enough time at work to give it more thought there ; )