I don't have enough to do at work, what should I do?

I was recently hired as an accounts payable tech at a small state agency, FT. Pay is decent and the benefits are awesome. I was hired mostly to take on the duties of the PT A/P tech, who is also the bookkeeper for an affiliated group. Anyway, I've been here a little over a month and I find that for days at a time I have no work to do. I process a few invoices here and there, act as receptionist and answer the phone when the real receptionist can't. As to keep from going nuts from boredom, I've reorganized our huge file cabinet, updated our agency directory, check through ancient project files, isted other co-workers, and whatever else I could find to do. I'm running out of busy work. The only time I'm *busy* is during the latter part of the week, when we are completing checkruns. It's very frustrating because my co-workers work hard all day, both PT and FT, while I'm struggling to find something to do. I was just told to "look busy", but my job satisfaction is ever decreasing. I'm not cool with getting paid to do nothing. Please help, what should I do???