I used to have the exact same problem. I did a lot of the suggested things, like creating D&D dungeons and creatures (largely just for fun) and doing crossword puzzles or other games. I also used to draw a good deal.
I also did a lot of practical things like pay bills and organizing financial records, writing grocery lists or things-to-do lists, and make little cheat-sheets for the job to post around my cube. You might even be able to do some homework, if you can avoid using your schoolbooks. Or, do clever things like photocopy the 6 pages that you need, and use those, since it’s easier to hide in with a stack of papers.
If you have any interest, you might also ask your supervisor if there’s something else for you to do in your spare time (presuming you’re already doing a good job overall). I wouldn’t ask this until you feel like you have the job down, and your stats are good, because otherwise you should probably spend your spare time trying to improve your performance or become more familiar with the job. Your supervisor might have some mindless busywork for you to do, and in my experience, getting in good with your boss generally means they let the occasional book or magazine slide – or they might give you a less conspicuous cube, where nobody can see what you are doing.