bored ALT, what do you do?

Are there any ALTs out there who have nothing to do in their free periods? You've prepared all the lessons, or the HRT did it for you, and have nothing left to work on. But then the principal catches you snoozing and tells you to prepare or to double check with the teachers about the lesson plan. Except the teachers aren't in the teacher's room, they're teaching their classes.
How do you make yourself look busy without hogging the computer?

Buy a notebook and practice writing Japanese, studying new vocabulary, etc.
Or, keep a journal in it: a work journal where you keep track of lessons covered in classes and how well they went over. ...or a personal journal of your stories from life and work. Not business related, but it might be fun for you to read years down the road.
Back to actually-business-related stuff, you can also buy books or print out pages on English grammar to help yourself better understand tougher questions from students. Or, maybe try memorizing student names. It's a monumental effort but really impresses teachers and students and makes class run smoother.