Attribute to Confucius (~500 BC). Still true 2500 years later.
When I was 7 or 8 years old on a miserable cold gray day in February my parents dropped my brother and sisters off at the Center of Science and Industry (COSI) in Columbus. http://cosi.org What a wonderful place! Our favorite of all exhibits was the interactive bike generator. Hop on and pedal. Flip on the switches and power your own lights. See how many watts you could make and for how long. All day long we kept coming back to try it again and again. We got it. Work, power, energy, electricity, volts, amps, watts, watt hours, renewable energy, conservation, limitations of the human body it is all included. Dead simple and yet brilliant.
For science teachers, PE teachers this should be a simple to build fun educational demonstration that could fit well into many different standards of learning objectives. This should also be great for anyone who wants to make a fun interactive exhibit or a science project.
To keep this simple start with an electric (assist) rear wheel direct drive hubmotor bike. This is a popular version of Chinese electric conversion kits. (Crystalyte, 9 Continent, Golden Motor (full size not mini)) (Pictured is the Crystalyte Roadrunner - 408 http://www.electricrider.com/crystalyte/).
For this conversion -
1)Place bike on stand (a simple stand for an Xtracycle - see my previous instructable http://www.instructables.com/id/Xtracycle-Stationary-Bike-Stand-for-Less/
2)Remove and Replace battery with Load (light bulbs and switches) (see following steps)
3)Pedal to generate electricity
Starting with another (not a rear wheel direct drive hubmotor electric bike) is likely doable but more complex. I will leave that as an exercise for the reader. The popular Bionx direct drive may work but as the Bionx motor contains a proprietary internal brushless controller it may (or may not) be more challenging to convert in this way.