Which human activities are of interest to researchers in their quest to invent viable human energy harvesting devices?
Walking, Running, Sitting
One technology is 
done with the use of piezoelectric devices, which produce voltages upon the application of a mechanical strain. A heel strike using this can produce 1 to 2 watts per shoe. Other technologies try to harness the motion of your body, which produces oscillations. The movement of your body also makes your clothes move, and some researchers are studying this as a way of generating electricity. You've already seen sneakers with LEDs in them that light up as you walk, courtesy of LA Gear and Fayva.
Waste
Urine is of interest because it can be used to make fuel cells. It can be hydrolized to make carbon dioxide and ammonia, and then the ammonia can be oxidized to produce nitrogen and water. This technology still has difficulties, because sodium hydroxide is required to make it, and this is a hazardous substance to transport. Also, a technique such as reverse osmosis is required to produce a yield of 0.5 to 1 watt of power.
Inertia
SEIKO Watch Corporation uses inertia and nanontechnology to create self-winding wristwatches that generate electrical power by using a weight that swings when you move. They are reliable and accurate. For more information, you can read about their Kinetic Brand on their website, and IEEE Spectrum has a good article on inventor Kunio Koike.
Thermoelectricity
These devices function with the use of your body heat. They use metals, semiconductors, and ceramics to create devices that exploit a heat differential. Some of them use the Seebeck effect to generate electricity. In addition to educational institutions and companies, NASA is pushing ahead with efforts to make this technology work, despite concerns that the human body cannot produce enough of a temperature differential between itself and other objects on Earth.
Inside The Body
Natural motion inside of the body is also being studied by researchers for possible energy exploitation. The heart is of course a no-brainer; studies are being conducted to use its natural motion to generate electricity that can be used to operate pacemakers. The flow of blood, especially inside the brain, is also of interest.
Make your Own Project
Interested in making your own projects?
If you wish to start with shoes, you can modify a pair of your own using stepper motors and a circuit as done by these intrepid folks. They include pictures, videos, and instructions.
How do backpacks, clothes, the flow of blood in the brain, and crowded subways help advance human energy harvesting technology? And why is the military interested? In part three, we will look at some of the projects researchers are currently working on.
References
Space.com on Human Energy Harvesting
Image Credits
Tokyo Train Station by Andy Weisner
Human Battery By Gizmag
Energy Harvesting Shoes by Christian Croft and Kate Hartman
Resources
An Introduction to Human Energy Harvesting
In this series, we learn about the basics of human energy harvesting. We discover what it is, the specific human activities that are useful for energy conversion, and discuss the current research being undertaken to make human energy harvesting devices available for popular consumption.