Things you can do with absolutely nothing...
Push your eyes for interesting light show:
(Amusement Potential: 1-5 minutes) See a variety of blobs, stars and flashes. Try to make out things - is your subconscious trying to send you a message? Can you control what you see by pressing different areas with different forces? Would it be possible to somehow see the same effects on TV? Or for that matter, watch TV with your eyes shut doing this?
See how long you can hold your breath:
(Amusement Potential: 4-20 minutes) Not that much fun, but it sure passes the time. Play with a friend, or try to beat your own personal best. Some tips: hyperventilate before hand, and stay as still as possible.
Try to not think about polar bears:
(Amusement Potential: 1-5 minutes) This is especially hard, because by trying too much, you remember what you were trying to avoid thinking of. If you try too little, you end up thinking about polar bears anyway.
(Amusement Potential: 1-3 minutes) Go ahead, scratch yourself now. Even if nothing itches, go ahead. Doesn't that feel pretty good?
(Amusement Potential: 1-3 minutes) What is pain? Why is it unpleasant? There's nothing physical about it - it's all in your mind. Plus, after pinching yourself for awhile, boredom will seem nice next to being in pain.
Try to swallow your tongue:
(Amusement Potential: 1-3 minutes) There's not much to say about this one. It is possible.
Look at something for awhile, shut eyes, study after image:
(Amusement Potential: 2-5 minutes) Another great time waster. It takes about 30 seconds of staring to create an after image, and the image is then viewable for about the same length of time. Fun to combine this one with pushing on your eyes.
Get yourself as nauseated as possible:
(Amusement Potential: 5-10 minutes) Best achieved by looking straight up and spinning around. Try to be so dizzy you can't even stand up. This is also entertaining due to the "makes boredom seem a lot better" effect (see "Hurt Yourself").