I run into a similiar problem with my part time summer job. What I have done a few times is find the complete text of a book online and e-mailed it to myself before going in, then open it up and read away. Great way to kill time while appearing to be doing something productive. Just don't get too into it, because it doesn't look good when your boss walks by your cubicle and you have your feet propped up on the desk reading.
I'm big on doodling. Yes, I said doodling, strange little pictures of things. I'd be a bit worried about a position with THAT much downtime. I'd suggest trying to get some added responsibilites if you want to go the productive route.