Some of these are pretty funny...
Scratch yourself
(Amusement Potential: 1-3 minutes)
Go ahead, scratch yourself now. Even if nothing itches, go ahead. Doesn't that feel pretty good?
Pinch yourself
(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.
Pretend to be a car
(Amusement Potential: 5-10 minutes)
Make appropriate revving noises in your head as you walk along and add a racing commentary as you pass strangers in the street. Use blinking eyes as indicators for extra authenticity
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").
Make a low buzzing noise
(Amusement Potential: 15-30 minutes)
Hours of fun in libraries! Keeping a totally straight face and looking nonchalant, make a low pitch humming/buzzing noise and see who reacts.